Add edX video ID section

Added the video_course.rst file to describe what course teams do after automated processing is complete.
@nasthagiri, @frrrances, @WatsonEmily , @lou-wang, @srpearce 
@mhoeber, @catong FYI.
No, and there will not be, Nimisha. This document is intended for use by course teams at member institutions only. Including a link to it in the completely open Building & Running guide would defeat the purpose.