Add section to dataflow troubleshooting docs around orphaned indexes

[chaas]

Addresses https://github.com/MaterializeInc/materialize/issues/27740.

Tips for reviewer

I confirmed this SQL works, but open to other recommendations. Is there any way to determine the objects that depend on the orphaned dataflow? I couldn't find anything in the catalog. It would be much more helpful if we could provide that list, so that users know what objects they need to recreate.

Checklist

• [ ] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [ ] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [ ] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [ ] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [ ] This PR includes the following user-facing behavior changes: