Add note that API docs (and migration guide) can be found on importlib_metadata docs

[CAM-Gerlach]

As discussed on the Python Discourse, it could be made clearer in the Using importlib_metadata documentation, and thus its CPython equivalent, that the API documentation (and migration guide) can be found on the importlib_metadata docs site, as many users are not necessarily aware of such.

Ideally, it would be included directly in the CPython docs either via Sphinx-Autodoc, manually or some other mechanism, given the linked version would be out of sync with what is actually included in the particular version of the CPython source, but this is a useful quick stopgap for now.