Docs: work through & fix Sphinx build warnings (& occasional error)

In #34 it was noticed the current docs source material contains numerous issues that throw warnings in the Sphinx build, which should be worked through and remedied. Some of these are indicative of issues in the code itself and not just documentation-related, which are the priority to fix, though it is probably simplest to aim to fix them all at once and if any emerge as more difficult to fix, raise a separate Issue to address later.

Specific issues, copied from console build process resulting in update from #34

$ make html                                                                   ─╯
python3 -m sphinx -b html -d ../build/doctrees   . ../build
Running Sphinx v7.2.6
[autosummary] generating autosummary for: add_cyclic.rst, advanced.rst, axes.rst, axes_plot.rst, bfill.rst, calculate_levels.rst, cbar.rst, cf_data_assign.rst, cf_var_name.rst, check_data.rst, ..., version_2.2.rst, version_2.3.rst, version_2.4.rst, version_3.0.rst, version_3.1.rst, version_3.2.rst, version_3.3.rst, versions.rst, vloc.rst, wrf.rst
building [mo]: targets for 0 po files that are out of date
writing output... 
building [html]: targets for 88 source files that are out of date
updating environment: [new config] 88 added, 0 changed, 0 removed
reading sources... [100%] wrf
/home/slb93/git-repos/cf-plot/docs/source/advanced.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/cbar.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.cf_var_name:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.cf_var_name:17: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/colour_scales.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/con.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.con:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/docs/source/cscale.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.cscale:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.cscale_get_map:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.cscale_get_map:8: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/cylindrical.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/download.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.find_pos_in_array:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.find_pos_in_array:13: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/gclose.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/gopen.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/gpos.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/graphs.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/gset.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/hovmuller.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/hovmuller.rst:6: WARNING: Title underline too short.

Example 10 - latitude-time
-------------------------
/home/slb93/git-repos/cf-plot/docs/source/internal_routines.rst:4: WARNING: toctree contains reference to nonexisting document 'timeaxis'
/home/slb93/git-repos/cf-plot/docs/source/levs.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/lineplot.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.mapaxis:11: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/mapset.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/multiple_plots.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.ndecs:7: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/polar.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.polar_regular_grid:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.polar_regular_grid:14: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/pressure.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.process_color_scales:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.process_color_scales:15: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/projections.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.regrid:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.regrid:12: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/reset.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.reset:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.reset:11: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.rgaxes:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.rgaxes:20: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/rotated_pole.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/rotated_pole.rst:32: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.set_map:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.set_map:12: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/setvars.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.setvars:32: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.setvars:67: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/stipple.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stipple:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stipple:21: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/stipple_plots.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/stipple_plots.rst:3: WARNING: duplicate label stipple, other instance in /home/slb93/git-repos/cf-plot/docs/source/stipple.rst
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stipple_points:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stipple_points:16: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stream:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.stream:31: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.supscr:8: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/training.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/traj.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/trajectories.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/trajectories.rst:140: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/docs/source/unstructured.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/user_defined.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/user_guide.rst:476: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/docs/source/user_guide.rst:485: WARNING: Inline substitution_reference start-string without end-string.
/home/slb93/git-repos/cf-plot/docs/source/user_guide.rst:1146: ERROR: Unknown target name: "colorbar".
/home/slb93/git-repos/cf-plot/docs/source/user_guide.rst:585: WARNING: duplicate label trajectories, other instance in /home/slb93/git-repos/cf-plot/docs/source/trajectories.rst
/home/slb93/git-repos/cf-plot/docs/source/user_guide.rst:664: WARNING: duplicate label graphs, other instance in /home/slb93/git-repos/cf-plot/docs/source/graphs.rst
/home/slb93/git-repos/cf-plot/docs/source/vect.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.vect:2: WARNING: Line block ends without a blank line.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.vect:62: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/vectors.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_1.5.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_1.6.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_1.7.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_1.8.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_1.9.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_2.0.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_2.1.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_2.1.rst:49: ERROR: Error in "image" directive:
no content permitted.

.. image::  images/scale1_original.png
   :scale: 52%




 ::

   Done
/home/slb93/git-repos/cf-plot/docs/source/version_2.1.rst:94: ERROR: Error in "image" directive:
no content permitted.

.. image::  images/cm.png
   :scale: 52%



 ::

   Done
/home/slb93/git-repos/cf-plot/docs/source/version_2.1.rst:160: ERROR: Error in "image" directive:
no content permitted.

.. image::  images/colourscales/BrBG.png
   :scale: 52%


 ::

   Done
/home/slb93/git-repos/cf-plot/docs/source/version_2.2.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_2.3.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_2.4.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_3.0.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_3.1.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_3.1.rst:92: WARNING: Title underline too short.

10. cfp.gpos(1) causes stray box lines
=====================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.1.rst:92: WARNING: Title underline too short.

10. cfp.gpos(1) causes stray box lines
=====================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.1.rst:218: WARNING: Title underline too short.

19. cfp.con - improved Z axis detection
======================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.1.rst:218: WARNING: Title underline too short.

19. cfp.con - improved Z axis detection
======================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.2.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/version_3.2.rst:182: WARNING: Title underline too short.

14. cfp.con - ugrid keyword changed to irregular
===============================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.2.rst:182: WARNING: Title underline too short.

14. cfp.con - ugrid keyword changed to irregular
===============================================
/home/slb93/git-repos/cf-plot/docs/source/version_3.3.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/cfplot/cfplot.py:docstring of cfplot.vloc:12: WARNING: Field list ends without a blank line; unexpected unindent.
/home/slb93/git-repos/cf-plot/docs/source/wrf.rst:2: WARNING: Explicit markup ends without a blank line; unexpected unindent.
looking for now-outdated files... none found
pickling environment... done
checking consistency... /home/slb93/git-repos/cf-plot/docs/source/advanced.rst: WARNING: document isn't included in any toctree
/home/slb93/git-repos/cf-plot/docs/source/colour_scales.rst: WARNING: document isn't included in any toctree
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/home/slb93/git-repos/cf-plot/docs/source/hovmuller.rst: WARNING: document isn't included in any toctree
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/home/slb93/git-repos/cf-plot/docs/source/training.rst: WARNING: document isn't included in any toctree
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done
...