[DOC] detection notebook - move plotting functions to `detection.plotting`

In the detection notebook 7 (added in https://github.com/sktime/sktime/pull/7284), @alex-jg3 has created a number of plotting functions that would be nice to have generally.

I think we should refactor the plotting functions and move them to annotation.plotting. There are also some existing functions in the latter, it might be useful to see if they can be merged.

As a recipe:

1. look at every block in the notebook 7, where matplotlib is used and something plotted
2. try to turn this into a function plot_something(param1, param2, ax=None) (where if ax is passed, no new ax is created but that is used)
3. put the function in annotation.plotting, and have the notebook import it from there instead of defining the plot in the notebook.