Add a document describing the content of documents

This gives details of the formats and semantics of fields included in
documents in the content store.

I wrote this partly in an attempt to ensure that I understand the
semantics of this, and partly to get some good documentation around this
for others. There are still some parts of this I'm uncertain about, so
I've marked them with TODO.
We haven't implemented it yet, but redirects will also be possible for regular items (eg to redirect a part of a guide that no longer exists).  Having said that, conversations yesterday may change how we handle that sort of thing.