Up-to-date API reference docs

I’ve just updated the manual, which was entirely outdated, with a completely new version based on doc comments, which is hopefully more likely to stay up to date. It does not cover everything yet (notably, the UI modules are still not documented), but it’s getting there.

I’m at a point where I think I can start promising API stability soon. There’s a few rough patches I want to work out before I start numbering, but I’m happy with the document and editor APIs, and the general shape of the schema and command APIs.