Add installation instruction to the manual for MatrixRTC

**Description:**

Hello there,

as Matrix is going away from classical TURN communication it would make sense to have a proper documentation at the official manual https://element-hq.github.io/synapse/latest/

Currently it's mostly trial and error and searching the web/guidance from other channel members.

Start by reviewing the official Synapse manual at https://element-hq.github.io/synapse/latest/ and identify where MatrixRTC installation guidance belongs. Research the current MatrixRTC and TURN setup expectations, then add enough documented steps that an administrator can follow the installation without relying on trial and error or external channels.

Written by the indexing model from the issue text.