docs: document uwebsocket module
It needs basic docs to explain that it's mainly used for webrepl and does not have a full websocket implementation.
See related: #2410, #4450.
RFC: Rename "websocket" to "uwebsocket"
This is similar to https://github.com/micropython/micropython/issues/4370, submitting a new ticket as that one has PR and hopefully can be closed soon.
Naming "websocket" module as such was pretty brave, as it's of course the default name for a module dealing with Websockets, and there should be tons of "websocket" modules around. MicroPython's module is pretty specific as it implements only data transfer handling, leaving handshake part to Python code.
"websocket" module was also not documented and described as highly experimental, with only intended user is webrepl subsys. So, I think here we can just rename it to "uwebsocket", without weak links, and at least include it in the list of modules in the docs, if not describe its API completely.