Reiterate the docs on entry points

[johnslavik]

Summary of changes

Hello! I've recently used entry points to plug-in with my serializer to RDFLib. Now, as importlib.metadata is part of the stdlib, most packaging tools other than setuptools support defining entry points. I therefore decided to reiterate some of the docs and gave a PEP 621-compliant example definition of them :)

LMK about any improvements you'd like to see here.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Granted push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[nicholascar]

Since we've changed to mkdocs, I've re-issued the changes in this PR in PR #3145