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.
I've moved the description of the `format` field nearer the top of the document, mentioned in it that the "gone" and "redirect" formats use a different set of fields, and added a TODO to the redirects field to mention that this field may only be valid for redirects, pending a decision.