migrate documentation to the new experimental python handler

[graingert]

bumping the mkdocs deps was needed to support jinja2==3.1.2 in https://github.com/encode/httpcore/pull/546/files however we're still using the legacy python handler and we should upgrade https://mkdocstrings.github.io/handlers/overview/#migrate-to-the-experimental-python-handler

[tomchristie]

What benefit/changes would upgrading the python handler have for the documentation?

[zanieb]

You can see the list of features for the new handler at https://mkdocstrings.github.io/python/ — it seems like this would only be useful if you want to include a generated API reference in the documentation though.

[tomchristie]

Okay, thanks for the suggestion.

If anyone's sufficiently motivated to open a pull request, and can demo which bits of the docs would change as a result then I'd be up for reviewing that.