Docs Cleanup

# Changelog
* Moved resources(configuration, extending, plugin reference, etc) to dedicated folder
* Updated plugin reference according to https://github.com/divio/django-cms/pull/2581
* Moved installation instructions to dedicated folder
* Moved parts of the old tutorial into installation/integrate and added reference to the new tutorial

# Issue-fixes
* fixes #2440
* issues 2 & 4 of #2150