First approach at a proper overview in the docs

Added a stub for editing basics doc that is focused on non-technical audiences.
Added to the 'how to contribute' to explain how to set up local environment for testing documentation changes.
A wholesale restructuring of the documentation.

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)
Merged django CMS tutorial into docs.

Merged the django CMS tutorial from
https://github.com/divio/django-cms-tutorial. The tutorial was originally
created by Kim Thoenen https://github.com/chive/.
Restructuring of how_to and topic sections
Synchronise docs restructure with support/3.0.x
More work on restructuring
Merge branch 'support/3.0.x' into docs-restructure2
More documentation moves to appropriate sections

* a lot of material has been moved from /topics to /reference and
  /how_to
* redundant sections (particularly navigation placeholders) removed
* /user section removed from index for now
Merge branch 'support/3.0.x' of https://github.com/divio/django-cms into docs-restructure2

Conflicts:
	docs/contributing/index.rst
Added back /user docs with TODOs.
Merge pull request #3557 from evildmp/docs-restructure2

Documentation restructuring