Summary:
- Split the PyPI docs per topic.
- Move everything to its own folder.
- Separate the `bzlmod` and `WORKSPACE` documentation. Some of the
  features are only available in `bzlmod` and since `bzlmod` is the
  future having that as the default makes things a little easier.
- Fix a few warnings.

Fixes #2810.

This generalizes the ability to populate the venv directory by adding
and additional field,
`kind`, which tells which directory of the venv to populate. A symbolic
constant is used
to indicate which directory so that users don't have to re-derive the
platform and version
specific paths that make up the venv directory names.

This follows the design described by
#2156 (comment)

This also changes it to a depset of structs to make it more forward
compatible. A provider
is used because they're slightly more memory efficient than regular
structs.

Work towards #2156