Backport docs changes

[jaraco]

In https://github.com/python/cpython/pull/102217, the docs were changed for importlib.metadata. Some of those changes need to be backported here.

[jaraco]

As I'm exploring backporting the changes, I'm finding it increasingly difficult to manage the consistency of the documentation across these two modes.

In particular:

• Sphinx is disallowing duplicate object descriptions for "importlib_metadata". The object description introduced in CPython conflicts with the one generated in the API docs here.
• There are lots of references to importlib_metadata/importlib.metadata that have to be kept locally-relevant.

For these reasons, I'm leaning toward just removing the documentation here and maintaining it solely in CPython.