Warnings when building the docs

Empieza ejecutando make html e inspecciona docs/index.rst y los archivos indicados bajo docs/source/ en busca de las advertencias notificadas. Confirma que la compilación de la documentación no produzca advertencias y, después, revisa la configuración de CI del repositorio para comprobar los fallos por advertencias, el Sphinx nit-picky mode y sphinx-lint; se considera terminado cuando estas comprobaciones se ejecutan correctamente en CI.

Building the docs locally yields a number of warnings:

% make html
mkdir -p build
PATH=./venv/bin:$PATH sphinx-build -b html -d build/doctrees  . build/html 
Running Sphinx v4.2.0
making output directory... done
loading intersphinx inventory from https://docs.python.org/3/objects.inv...
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 20 source files that are out of date
updating environment: [new config] 20 added, 0 changed, 0 removed
reading sources... [100%] source/writing_stubs                                                                                              
/Users/jelle/py/typing/docs/index.rst:86: WARNING: Title underline too short.

Type-Hint and Stub Integration
----------------------
/Users/jelle/py/typing/docs/index.rst:86: WARNING: Title underline too short.

Type-Hint and Stub Integration
----------------------
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/jelle/py/typing/docs/source/annotations.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/basics.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/dependencies.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/faq.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/inference.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/introduction.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/type_compatibility.rst: WARNING: document isn't included in any toctree
/Users/jelle/py/typing/docs/source/type_system.rst: WARNING: document isn't included in any toctree
done
preparing documents... done
writing output... [100%] source/writing_stubs                                                                                               
/Users/jelle/py/typing/docs/source/generics.rst:290: WARNING: undefined label: advanced_self
/Users/jelle/py/typing/docs/source/generics.rst:642: WARNING: undefined label: casts
/Users/jelle/py/typing/docs/source/protocols.rst:510: WARNING: undefined label: async-and-await
generating indices... genindex done
writing additional pages... search done
copying static files... done
copying extra files... done
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 13 warnings.

The HTML pages are in build/html.

Build finished. The HTML pages are in build/html.

We should fix these.

Additionally, this should be caught in CI. Some changes to make:

• Make builds fail on any warnings
• Turn on Sphinx nit-picky mode to catch more issues
• Install sphinx-lint to find additional issues

I'll send a PR to fix the warnings but I'm not currently planning to work on the CI improvements.