Course re-run section and refs to it in others

@explorerleslie, @talbs please check for accuracy and completeness
@WatsonEmily please circulate to PMs for level of detail, other likely questions to address, format
@mhoeber , @srpearce please consider this a developmental/copy edit
@sarina, @nasthagiri if you have time I value your input also!

Is "Course Run" synonymous with "timeframe" in our documentation?  Code-wise, the Run is just a string value, and doesn't necessarily have to be a timeframe.