Document how to manually rerun periodic Prow jobs

[wallrj-cyberark]

Motivation

There was no documentation on how to manually retrigger periodic Prow jobs.
This came up during the cert-manager 1.21 release preparation: after merging
a .trivyignore update (cert-manager/cert-manager#8983) to fix trivy scan
failures caused by GO-2026-5932, we needed to retrigger the periodic trivy
jobs rather than waiting for their next cron cycle.

Changes

Adds a "Rerunning periodic jobs" section to prow/README.md covering:

• Why you might need to do this (e.g. pre-release reassurance after
  dependency updates)
• Prerequisites (GCP IAM role, kubectl cluster credentials)
• How to find ProwJob IDs (via the Prow UI or API)
• The kubectl create -f rerun command
• A link to the cluster definitions in cert-manager/infrastructure

[cert-manager-prow]

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