Add options for partial documentation builds

[greschd]

Add the following options to run a partial doc build, controlled
via environment variables:

• PYACP_DOC_SKIP_GALLERY=true: do not build the examples
  gallery
• PYACP_DOC_SKIP_API=true: do not build the API reference

Add a quick variant of the documentation build in CI which only
builds the main pages (no API / examples). This build runs in parallel
with the full documentation build, to provider quicker feedback
on errors.

Add docstrings to the enum classes.

[greschd]

Note 1: the expected checks in the branch protection will need to be updated.

Note 2: we could also add builds that run only the examples / API separately.

Note 3: I originally tried to improve doc build speeds in CI by switching to a theme
that doesn't create separate pages for attributes, but this caused some warnings
that I couldn't easily resolve.