[docs] moving resources to separate folder, updating refs
[docs] Updated plugin reference according to https://github.com/divio/django-cms/pull/2581
[docs] new installation subdirectory + overview
[docs] add configuration and setup instructions to installation
[docs] Add reference to the new tutorial
[docs] moved parts of the old tutorial to the integrate part
removed old tutorial
[docs] updated index, added tutorial reference
Merge branch 'develop' into docs/cleanup
[docs] added file permissions section
Merge pull request #2043 from Chive/docs/file-permissions-plugins

Document #1772
Merge pull request #2748 from Chive/docs/cleanup

Docs Cleanup
[docs] explains `with` tag usage in templates
Merge pull request #1027 from Chive/docs/plugin-context

[Docs] plugin reference should cover provided contexts.
backwards compapatible mixin classes