Document Story specific viewer features.

Over the past few months we've been adding a few Story specific viewer features, that we should document:

- `(get|set|on)DocumentState`
- `education` capability and `canShowScreen` messages
- viewer to document `selectPage` message
- document to viewer `selectDocument` message
- To be considered: how to control the story audio experience in a native viewer that can enable audio to autoplay

They should appear in the [public documentation](https://github.com/ampproject/amphtml/blob/master/extensions/amp-viewer-integration/amp-doc-viewer-api.md), and in the internal version (that is, I think, a copy paste of this one).

cc @ampproject/wg-stories @CrystalOnScript