feat(docs): add inheritance diagrams & explain overrides

[lwasser]

This is a pr to begin discussion (when we are ready) about adding schematics for each inheritance pattern in our API (for key objects).

here, ive added the sphinx inheritance extension which uses graphiviz on the backend to create a diagram based on our inheritance patterns.

We can customize most aspects of the diagram and it will update as/if we update inheritance patterns. this might be a nice supplement to our API docs. I've also removed some deprecated client endpoints here that we no longer have in our API. Just so the docs build as they should.

Have a look at the diagram below (or the docs build in ci) and let me know what you think about doing this for the main classes - Athlete, Activity, (others?)