Clean up Prometheus docs

The Prometheus docs are scattered in various places and when I tried to understand things, my brain hurt.

The main 3 categories are:

• How to set up Prometheus itself
• How to enable the Prometheus integration service
• How to make Prometheus gather metrics (and what metrics are supported)

Things are getting interesting when Kubernetes comes into place and the complexity of setting up things sky rockets.

I think the first thing we need to do is restructure some stuff, mostly under project/integrations/prometheus/.

cc @joshlambert