Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
What do you mean by "live course" here? I've heard "live" used in the same way as "released to students", but you can access the instructor tab before any courseware has been released to students.