When fronting the playbook's integrated Traefik reverse-proxy with
another reverse-proxy (e.g. nginx), WebSocket traffic needs to be
forwarded to LiveKit Server at the `/livekit-server/` path for Matrix
RTC calls to work.

Adds a `/livekit-server/` location block to the nginx reverse-proxy
example, a section to the Matrix RTC docs explaining the requirement,
and cross-links between the two.