"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.
Doing a find-all shows that 'frontend' is used most of the time (although Safari just auto-corrected that to "front-end"). There are 6 occurrences of 'front-end' elsewhere in the docs. Let me know a decision and I'll make the docs consistent.