Add documentation about when to enable merge trains
What does this MR do and why?
We have discussed when to enable merge trains for gitlab-org/gitlab
in a recent broken master wheel of misfortune meeting.
This MR adds a summary of this discussion in the pipelines development documentation for all to see.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.