"Editing Basics" first pass

I have more documentation/screenshots to add (covering things like the "Page" menu when editing a specific page, adding content/plugins, etc.) but this at least gets rid of the "TODOs" in the docs.
Perhaps link to the soft root explanation: http://docs.django-cms.org/en/support-3.0.x/topics/menu_system.html#soft-roots.

Also missing a full-stop at the end of the sentence.