"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.
See http://docs.django-cms.org/en/support-3.0.x/contributing/contributing.html#inline-markup for how to mark up this sort of thing. I'd consider "Pages..." to be an example of "the name of an available option in the Admin" (I think).