docs: make Stage context explicit in promotion step examples

[MuhanedYahya]

Issue Reference

Closes #6275

Description

Several promotion step reference examples used ${{ outputs.alias.field }}
without any surrounding YAML context. This created an unstated assumption
that the reader understood the snippet to be inside a Stage's
spec.promotionTemplate — not inside a (Cluster)PromotionTask, where
the correct syntax is ${{ task.outputs['alias'].field }}.

As noted in the original review on #6273, the fix is to make the context
explicit in each affected example rather than leaving it as an assumption.

Checklist

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• The linked issue has no blocking labels (kind/proposal,
  needs discussion, needs research, maintainer only, area/security,
  size/large, size/x-large, size/xx-large).
• I have added or updated tests as appropriate.
• I have added or updated documentation as appropriate.

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