generator: document monitoring generator (features, dev, etc)

Created by: bobheadxi

Part of https://github.com/sourcegraph/about/pull/2000 - we currently do not do a great job "marketing" the benefits of the monitoring generator, which makes it seem more like an unpleasant restriction than a "oh hey I get tons of stuff for free by using this" deal.

Also add back this line: https://github.com/sourcegraph/about/pull/2000/files/d7d462a097ac797ceee39265235a1a2e8690cee0#discussion_r523022580