Inline documentation for Zed commands.

Ok, this is coming along ;-)

* it reads the 'doc' property if present to describe the commands. I put in a few but should really flesh it out.

* it does some nice word-wrapping on long lines

* has foldable category names (however I'm a bit disappointed with the markdown mode, it doesn't highlight the section headers to distinguish them from the rest of the document.

I haven't done the "type enter to invoke this command" thing, still working on it.