Clean up the docs

[keevindoherty]

Doxygen setup right now is not super nice (not well formatted or linked with GTSAM docs).

If we can install GTSAM on the Docker image that builds the documentation, Doxygen should be able to figure out inherited types from GTSAM.

Also might consider playing with Doxygen configs to make the documentation generally a little "nicer".

[keevindoherty]

Consider also modifying the documentation build triggers so that the docs don't have to build so often?