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.
It might be worth calling out that if they do not create cohorts before the course begins, and people go to the discussion forum, then those people will not be automatically assigned to a cohort (i.e. are in the 'general' cohort, and not a specifically assigned one). You might want to check with product on it, but I can see this going wrong (i.e. people creating their cohorts after the course starts, then it doesn't work as they expected).