handbook: distribution: minor observability changes

Created by: slimsag

I linked a bunch of people to https://about.sourcegraph.com/handbook/engineering/distribution/observability but it's a bit barebones right now and could use more direction on where to go:

This adds that direction:

And additionally points people to some additional reading not mentioned in the developer guide, like (1) how high-level alerting metrics work and (2) the difference between Critical and Warning alerts and how admins consume those.