Lobbies docs

This is a rough mock up, there's several small things, like the parenthesis on the c++ side of Create Lobby.

I'm not entirely sure this is the way you're going to want it done, mostly due to the Lobby subsystem having less bound delegates exposed, relative to say the Session subsystem.

Let me know what changes you'd like to see and if you'd prefer to talk to it over on voice chat, we can do that.