✨ [FR] Add a directive that outputs the configured change note categories

[webknjaz]

I was writing contributing docs about Towncrier new fragments and realized that it's PITA to maintain a copy of pyproject.toml entries and not to forget to update entries in Sphinx documents on any change.

So I think it'd be a great addition to what the extension already does. Although, it's not urgent and I'd rather postpone implementing it until the dev/test/packaging/publishing tooling/infra is set up at least. For now, I just wanted to record the idea on the tracker.