Better documentation

[eric-unc]

The documentation is now pretty good, but it's still something of a mess. So there's more to be improved:

• documentation on basics of the language
• (including) type info
• examples
• get a proper theme for the documentation. I think it looks good, but also it looks a bit plain, and the "Improve this page" on the bottom is a bad idea when it's supposed to be automatically generated.

This is mostly low hanging fruit; a longer term goal is to have the whole documentation engine written in pure Psil, but we're not quite there yet.