docs: port admin guide to sphinx

Change-Id: I1ce8ccfc440f4856a5b6db331fbd8695c09986c5
Improve BI and reporting docs

- detailed explanation of current Graphite gauges
- specific suggestions for tweaking BI queries
- describe the process for requesting new supported queries

Change-Id: I7336bd9fe365d5d5be20fe515e10f33ea71a1a37