Document API for Connection Server (aoo.sonobus.net)

[mendeni]

It would be helpful to have the API connection server documented to support development of services outside the SonoBus client.

For example, this would allow an external query to see who is currently online and create notifications accordingly. I currently find myself periodically checking public sessions to see who's currently online. This is inefficient and can be distracting to session players.

More context/details in Discord conversation.

/cc @essej