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 had talked about including the title, and additionally both a `web_url` and `api_url` for the items.  The latter 2 are pretty cheap to generate, and could also be easily generated by the consuming apps.  We'll probably want to add them to the responses at the point we make the API public, so it may make sense to just add them now.