Write and publish docs

We should have docs for at least all public APIs, and published rustdocs linked from the README.

It's not super straightforward to use this repo with crates.io (you need a local checkout for getting to the target JSON file, see #1). docs.rs automatically publishes crates on crates.io; I'm not sure if we want to publish the crate for the sake of using that (+ getting into things like crater runs?), or just self-host docs. (I run enough websites that I'm not going to be sad about self-hosting docs somewhere....)