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.
@explorerleslie , we get a lot of feedback through the PMs that people are unable to set up new course-wide discussion topics because they don't remember the comma and the save silently fails. I will combine these steps but we may find that these steps are not sufficient to help people succeed.