[docs] Expand a bit on the basics of a buildbot policy

We don't have much publicly documented about how build bots are maintained, and who has what responsibility with regards to the bots themselves and failures they report.

This change adds some basic framing on the topic. It is intended to simplify document long standing practice, and to avoid introducing any new policy elements. I do want to propose a couple of changes, but those will happen with their own review and broader discussion.

Differential Revision: https://reviews.llvm.org/D112755