Doc changes (MB style and API documentation)

This is a branch with a couple of doc changes.

Currently one small style change so the style works for readthedocs:
https://python-musicbrainz-ngs-jonnyjd.readthedocs.org/en/doc_changes/
and listing all Error/Exception classes in the API.

I might add more changes later on, but this can be merged and/or reviewed at any time.