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.
This sentence is very long, and IMHO, this reads better with some commas "assigned to an auto cohort group, if you have configured one, or to the default cohort group if you have not". But I don't know what is grammatically correct or preferred.