Document way to do notice compatibility routes with Nginx reverse-proxy, fixes #2900

This MR does an alternative of removing the notice compatibility routes from the Pleroma backend and instead documents how to do such with Nginx reverse proxies. Currently, I only modified the installation/pleroma.nginx file. but if it's better in the documentation site, I can do that as well. Special thanks to @ilja for noting how to do this and to @tusooa for additional help.