Inline documentation for Zed commands.

Ok, this is coming along ;-)

* it reads the 'doc' property if present to describe the commands. 

* 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).

* categories are all folded by default, allowing you to see an overview at first and drill down to the information that you want.

* allows you to invoke the nearby command by pressing enter