gh-93963: Document importlib.abc deprecations

[hugovk]

Follow up to #93965 (and re-using its issue number, let me know if we need a new one).

Document the deprecations made in issue #93963 / PR #93965.

• Issue: Officially deprecate and remove abcs in importlib.abc moved to importlib.resources. #93963

[warsaw]

How does this relate to #93611

[hugovk]

Let's ask @encukou:

Some new deprecations were added in #93965, but not documented.

Your #93611 does some restructuring, and given you want to limit its scope, should these deprecations be documented before or after that PR is landed?

[encukou]

Sorry for missing that notification, and thanks Łukasz for merging both!