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 also might be worth highlighting that if you're doing a hybrid approach, you want to make sure the people you are assigning manually are assigned before the course opens, so they don't get accidentally auto-cohorted when the course opens. It's not a huge deal if they're moved after, except the student will see different posts available to them before and after.