Documentation improvements

[tbenthompson]

• Liz or Marc should look through and edit/improve the Readme.
• We should add some useful instructions to the Sphinx page. Possibly installation and quick start. Alternatively, it can just be a copy of the main Readme.
• Remove glm_benchmarks from the public API in Sphinx.
• Fix strange Sphinx format the API page hierarchy

[tbenthompson]

@MargueriteBastaQC I think you've fixed some of the issues here!

[MargueriteBastaQC]

Moving to done for now. API page hierarchy could still be improved (would be nice if the dropdown from the API reference was just each of the external facing API functions/classes), but looks like this might not be possible until this sphinx issue is fixed: sphinx-doc/sphinx#6316