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.
Short answer, is that we should be doing what @tekin suggests, we're just not going to do that yet.  At the point where we start surfacing the needs themselves on GOV.UK we can than transition to using the tags mechanism for them.  Until that time, we'll stick to the existing mechanism.