Add human-readable reference for WebSocket channels/states

Currently it's kind of hard to wrap your head around all the different states and channel messages that websockets might send. Perhaps this is something that isn't really meant for the channels docs, but it seems like there should be a reference of all the different messages that you can route on for websockets.

For example, this [blog post](https://www.machinalis.com/blog/introduction-to-django-channels/) references a `websocket.keepalive`, which I don't see documented anywhere. However, I don't know if that is because it is deprecated, or just not documented. It would be great to have a canonical set of possible channels/messages you can route on.

Another useful thing would be a full topical guide on Websockets and how to think about them in regards to channels. That might be a good topic for another issue, though :)

Start by reviewing the existing Channels documentation and the current WebSocket routing and message behavior. Verify whether websocket.keepalive is supported or deprecated, then define the canonical set of routable messages and states; a separate topical guide should remain out of scope.

Written by the indexing model from the issue text.