Mention API ref & migration guide in importlib_metadata See Also

[CAM-Gerlach]

Per #400 and as As discussed on the Python Discourse, this clarifies the Using importlib_metadata documentation (and thus its CPython equivalent) that the API documentation (and migration guide) for the latest version of importlib.metadata` can be found on theimportlib_metadata`` docs site.

Fix #400

[jaraco]

On further consideration, let's try the links. If they become onerous to maintain, we may replace them later with a less specific reference.