EPIC: Expand documentation

[avirshup]

Documentation is cleaned up and expanded.

• [x] Clean up main page - 7c216b0420f2a11d128b102f91b54b035f25734b
• [ ] Clean up and sort auto-generated API docs - WIP (finish sorting, check links)
• [x] Expand data structure documentation 7c216b0420f2a11d128b102f91b54b035f25734b
• [x] Add "features" page 7c216b0420f2a11d128b102f91b54b035f25734b
• [ ] Add "screenshots" page
• [ ] write recipes for calculations / properties / dynamics
• [ ] write recipes for various atom selections - document new atom selection features
• [x] theochem/horton has great docs - https://theochem.github.io/horton/2.0.1/index.html - we should emulate them.