Draft: Add QA specs for the prevent duplicate pipelines feature

What does this MR do and why?

• General idea from the Epic: Create at most 1 pipeline when in the context o... (gitlab-org#6044)
• Full implementation POC and screenshots: POC: Solve duplicate MR pipelines (!237729 - closed)
• First step: Add skip_branch_pipelines_for_mrs project setting (!241086 - merged)
• Second step: Expose skipping branch pipelines project setting (!241641 - merged)
• Third step: This MR

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