Document inject_policy strategy for pipeline execution policies

What does this MR do and why?

Document inject_policy strategy for pipeline execution policies.

References

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• Issue: Enforce Custom Stages in Pipeline Execution Pol... (#475152 - closed)
• Backend MR: Inject custom pipeline execution policy stages (!178199 - merged)
• Frontend MR: Add inject_policy as default pipeline execution... (!178715 - merged)

MR acceptance checklist

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Related to #516179 (closed)