Mention Tor Weather in the docs for relay operators

The docs for relay operators (https://community.torproject.org/relay/setup/post-install/) mention the https://uptimerobot.com service to help monitor a relay by sending a notification if the relay becomes unreachable. The tor weather service (re-launched) should accomplish that also (plus it has a few additional options). It'd be good to add a mention of it in the docs.