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.
Just to be clear, this is an optional practice? Wanted to make sure people know that this isn't required, but may be useful to distinguish different conversation types. This use case assumes that a minority of discussions are private. Other use cases include: 
- the entire course is cohorted, and there are no 'public' discussions (i.e. no need to distinguish that it is private); and
- the majority of the course is cohorted (i.e. it would be easier to distinguish the 'public' discussions, rather than the private ones)