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.
Info on using the csv of student profile info to get the cohort assignments is still TBD. My assumption is that course teams would be responsible for keeping up with any manual assignment work as long as any enrollments continue after the start date, and sorting the CSV by the cohort column would assist them in finding any students who do not have an assignment yet or who are in the default cohort. Is that essentially what you would want to see documented, @cahrens ?