Update docs

[srithon]

The documentation was pretty messy, and the whole target documentation vs. functional documentation thing was confusing. This PR greatly cleans up the functional documentation and adds a Feature Overview section to the main README, so people don't need to search through the example to figure out what the features are. It also replaces the target documentation with a "planned features" file referencing the things that differed from the functional documentation.

[srithon]

I ended up fixing a bug while updating the documentation :facepalm: