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 pretty certain we will be including the title at the very least when expanding out the tags. The minimum we want to enable is for frontend apps to be able to generate a list of links to these tags without having to make additional API requests.