Documentation: improve performance by bypassing Peertube for Websocket and BOSH

[JohnXLivingston]

In order to have a one-click installation of the plugin, I have to use Peertube to proxify some resquests to the Prosody server.

To improve performances, it is possible to bypass Peertube by addind some specific nginx configuration.
This can only be done by admins who have root access to the server.

So we should add a section in the documentation, for some scalability tuning, and document how to bypass Peertube for Websocket and BOSH connections.