Docs main rework

**NOTE:** this is in-progress because it also contains a commit that's in #307 and I got lazy. I assume this will shake itself out when we merge that one.

Reorganization of main page to try to put most important info at top.

Also removes toctree on this page, meaning it and the 'guide' pages will have a left-hand menu with section titles instead of API doc links.

The API docs are still there, just linked to directly from the dev setup page.