Documentation restructuring, work in progress

Feedback and comments please. May have some stray bits from the develop branch in it.

The existing content (some of which will have to be reworked to fit better) is
now in the following main sections:

* Getting started (introduction): step-by-step tutorials and guides to
  get you up and running
* How-to guides (how_to): guides covering more advanced development
* Key topics (topics): explanations of key parts of the system
* Reference (reference): technical reference for APIs, key models and
  so on
* Development & community (contributing)
* Using django CMS (user): guides for using rather than setting up or
  developing for the CMS
* Release notes & upgrade information (upgrade)