gh-134740: Document distribution discovery in importlib.metadata (#134751)

Document the following items listed in `__all__` but missing from
documentation:

- `distributions()`: mentioned in doc strings as well
- `DistributionFinder`: mentioned but didn't have it's own :class:
  entry
- `DistributionFinder.Context`: mentioned but didn't have it's own :class:
  entry)
- `Distribution.discover()`: mentioned in doc strings

Closes #134740
Partially addresses #110937

---------

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>
Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>