Documentation restructure

[Daniele Procida]

I've completed some work on the documentation. There is more to be done, but I think it has reached a stage where it can be merged into the support/3.0.6 branch.

<https://github.com/divio/django-cms/pull/3557>

The documentation is now arranged thus:

* 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've also:

* removed a great deal of duplication
* merged in the django CMS tutorial from <https://github.com/divio/django-cms-tutorial>

Daniele

[Martin Koistinen]

Wow, this sounds great! I look forward to seeing it.

[John-Scott]

The 'contributing' docs reference the https://twitter.com/djangocmsstatus account but there hasn't been any activity there in a year and half. Should we remove the reference or do y'all plan to resurrect this at some point? The same sentence in the docs also mentions 'Hudson' rather than Travis but if the Twitter account is a dead parrot, I'll just remove the entire sentence.