Improve websocket docs

[Martinsos]

• Open up with a bit of general intro to websockets, what are they, when are they useful, and point to some useful external docs to learn more.
• Mention that you will likely want to have multiple rooms, since otherwise, everybody sees all messages, and then give some guidance on how to do that -> it is not a Wasp feature yet, but still, this will be valuable to people. Find a good balance between explaining it all ourselves and linking to external docs.