Add some custom css to improve navigation. Also added some internal links, code cleanup

After merging the most recent upstream, I did some work to add some styling that displays more navigation in a meaningful way. This makes it easy to navigate deeper while maintaining a visual distinction between the different documentation sets.

Also included are some cleanup of extraneous formatting on the data object docs and some internal page links on api/overview.rst (added separate subheaders for object lookup and search endpoints)