docs: clarify Worker deploy runbook

[JimCollinson]

Summary

• document the GitHub Actions steps for preview and production Worker deploys
• call out the required production environment approval for both deploy workflows
• explain where to find the workers.dev preview URL and how to set PREVIEW_URL for smoke tests
• add a root README pointer for Worker-change deployment handoff

Verification

• cd worker && npm run format:check
• python3 scripts/adr-governance.py

Docs-only change; no Worker code or workflow behaviour changed.