Improving docs

[ivanpauno]

• Fix DeclareLaunchArgument reference in architecture.rst
• Fixes in sphinx conf.py:
  • Deleted html_static_path (there's no _static folder)
  • Add options to make class/constructor docs nicer (don't merge init signature with the class name, document init and the class separately).
  • There are other issues like methods with type hints being rendered horribly.
    IIUC the only way to fix that is to provide a custom .css file, which I haven't added (see this issue).
• Add python and xml examples in DeclareLaunchArgument.
• Make pytest run doctests, I needed to add some extra conftest.py files to make this work.

My idea is to add simular examples to all actions/substitutions, and then revamp the architecture docs (I'll probably split that in a more basic intro, reference documentation of all actions and substitution, and then the more detailed architecture).

[ivanpauno]

We will actually have the generated docs when ros-infrastructure/rosdoc2#28 is merged.

[hidmic]

I like where this is headed.

[jacobperron]

LGTM with CI

[ivanpauno]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows