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.
This doesn't work for me. That is, I don't see any options for the Default Timed Transcript field after deleting the Default Video URL.