Add human-readable reference for WebSocket channels/states

[ericholscher]

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 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 :)

[ericholscher]

This might be solved with this actually: https://channels.readthedocs.io/en/stable/asgi.html?highlight=linear#connection -- might be useful to break that out into the channel docs as well, though.

[andrewgodwin]

There are reference docs as you linked, but they're specification-style and not particularly user friendly; a shorter page with more human explanations and maybe a diagram would probably help.

(and for reference, keepalive was indeed deprecated and removed a while ago now)