observability: clean up monitoring dev docs
Created by: bobheadxi
This is a companion PR to https://github.com/sourcegraph/sourcegraph/pull/16965. Copied from that PR, the goal of these changes is to define a new split in our monitoring docs:
- docs.sourcegraph.com: general monitoring dev docs in the monorepo (adding monitoring, finding monitoring in the code, etc)
- about.sourcegraph.com: sourcegraph-specific monitoring dev docs (e.g. cloud dashboards, using opsgenie, other usage, etc), monitoring philosophy docs (pillars, architecture)
Closes https://github.com/sourcegraph/sourcegraph/issues/16938