Add guidance for determining unplanned stops

What does this MR do and why?

Adds guidance to the avoiding required stops documentation about who to contact if we may need to consider an unplanned stop. It also clarifies that we may escalate to product leadership to make a decision about whether to declare a required stop when it is unclear whether one is actually required.

Related to gitlab-org/enablement-section/discussions#98

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.