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.
I'm not sure we really want course authors to set the `Downloadable Transcript URL` field if edx-video-ids are used.