Make API Reference page tell me something I want to know

[leifwalsh]

Linking yourself through this page is sort of the fastest way to the docstrings, but it's kind of a stark thing to land on. We should put some text here that welcomes the reader and points them to the right next thing to click: https://marbles.readthedocs.io/en/stable/reference.html

Also, maybe the module docstring summaries, maybe also go one level deeper on the toctree?