Warn about pre/post-stage on general CI debug page

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What does this MR do?

Both Merge pipeline not created until an additional ... (#439220 - closed) and https://gitlab.zendesk.com/agent/tickets/524429 ended up having this easily overlookable detail as the core issue, so I think we need to highlight it more strongly. People won't be looking at the https://docs.gitlab.com/ee/ci/yaml/#stages keyword definition when they experience the problem that no pipeline is being run because of it, so I think simply duplicating the sentence into the general CI debugging page under the Jobs or pipelines don’t run when expected header is a good spot to highlight it.

Related issues

#439220 (closed)

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