Draft of video student-facing documentation

@mhoeber @lamagnifica @catong @explorerleslie This is the first draft of student-facing documentation for the video player. It's not much more than a list of features and a link to DemoX; do you think it needs anything else?

Also, before anyone comments on it: I wanted to make sure all the descriptions of video player controls were parallel; since not all of them could have started with imperative verbs, I went with complete sentences. :-)

Oh, one more thing: I added a copy of the formatting cheat sheet to this directory for convenience; no need to review it, of course.
YouTube seems to call it "default mode", which resizes automatically based on the size of the browser window. https://www.youtube.com/watch?v=qY173FCff_c