docs(medcat-trainer): Migrate docs to Mkdocs. Update Navigation

[alhendrickson]

See new version on https://cogstack-platform-toolkit--353.org.readthedocs.build/projects/medcat-trainer/en/353/

Included the suggestions from previous docs PR.

Tried to leave the docs content mostly as is for now.

Two notes:

1. this doesnt have the python API generation as on https://docs.cogstack.org/projects/medcat-trainer/en/latest/autoapi/index.html . If it's essential we can add it before merging, else I'd do it later - definitely plan to add to medcat (library)
2. Sticking with Mkdocs/Material for mkdocs, instead of Zensical, even though there's now a big warning as of last week printed - the plugins arent there yet for anything else. I'm assuming we 100% need that api docs for medcat (library), and the others wont support it. Zensical looks slick though.

[mart-r]

Comment regarding the pyproject.toml for these docs.

Also, as for the API reference, I think it would be good to have it for the client at the very least.

[mart-r]

Contextually, this seems documentation for medcat-trainer, specifically, rather than the entire cogstack ecosystem.
Or am I missing something? I.e is this creating the entire thing? If so, perhaps it should be in a different folder?

[alhendrickson]

Done - thanks! There was a lot of copy+paste involved in making this PR :D

[mart-r]

Same as above

[alhendrickson]

Done