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. :-)