Update release promotion checklist for standalone chart

[danehans]

Summary

• add hack/release-staging-digests.sh to collect release digests for promotion
• include all promoted artifacts in one place: charts/inferencepool, charts/body-based-routing, charts/standalone, epp, and bbr
• update the release checklist step for k8s.io promotion to call out charts/standalone explicitly and require adding a new section when needed

Why

charts/standalone is now part of release artifacts and was easy to miss during manual k8s.io image promotion updates.

Validation

• Ran: MAJOR=1 MINOR=4 PATCH=0 RC=1 ./hack/release-staging-digests.sh
• Verified script outputs digest mappings for all five artifacts.

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[k8s-ci-robot]

@danehans: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-inference-extension-verify-main	3c4cb8a	link	true	/test pull-gateway-api-inference-extension-verify-main

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[danehans]

/retest