Improve API docs
Currently, the API docs are out of date in places. In addition, just giving an example JSON return value does not indicate what all of the types of the keys may be (particularly whether they can be null or not). It would be nice to document all of the entities in entities.rb (also webhook and system hook objects) with proper type descriptions and such (also noting that repository keys are all deprecated, etc.). The API docs can then just reference the entity they return which would make the docs way easier to read without giant blocks of JSON in between all of the method calls.
I've been going through the docs and code to figure out how to expose the API in rust (for this library), so I have lots of stuff already done, but places for things like Option<> are fuzzy since there's not much indication for what can be null. If we can figure out a format for documenting the return objects, I can work on putting the Rust types back into markdown for documentation here (and hopefully they can be kept up-to-date much easier!).