`room_invite_state_types` was inconvenient as a configuration setting, because
anyone that ever set it would not receive any new types that were added to the
defaults. Here, we deprecate the old setting, and replace it with a couple of
new settings under `room_prejoin_state`.
https://github.com/matrix-org/synapse/tree/develop/docs/sphinx doesn't seem to really be utilised or changed recently since the initial commit. I like the idea of exportable documentation of the codebase, but at the moment after running through the build instructions the generated website wasn't very useful...