Move monitors docs to the Reference section

Part of an epic to move reference material to the reference section.

1. Create a folder called Service Assurance in the Reference section.
2. Move service monitors folder from Operation guide to "Service Assurance" section of Reference section.
3. In "Operation" section, create "stub" service monitors adoc file that includes a link to the Service Monitors in the Reference Section.

Next is to determine user-focussed actions that we can provide "recipes" for. Bring the examples from the existing service monitors documents into the "Operation" guide.