Add detail on how to coordinate Monitor SRE shadow rotation

Why is this change being made?

In becoming the second SRE shadow for the Monitor stage, I felt pretty confused about what needed to happen to move things forward. This MR adds clarity to the process and is intended to put a little more ownership on the engineer. They might be a "shadow," but they're certainly the one in control. I wanted to emphasize that a little in the language here.

Maybe it could be more, even?

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what