Documentation restructuring

Feedback and comments please. 

The existing content is now in the following main sections:

* Tutorials (introduction): step-by-step tutorials 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 (in progress; not in index)
* Release notes & upgrade information (upgrade)

I have also:

* removed a great deal of duplication
* merged in the django CMS tutorial from https://github.com/divio/django-cms-tutorial