Document DM notification config and deploy checks

[nmayalais]

Summary

• document DM-first notification routing and public escalation behavior
• list the new optional notification config keys in setup docs
• add the CI-aligned predeploy check gate to Apps Script deployment guidance

Validation

• git diff --check
• documentation grep confirmed the new config keys and CI parity checks are documented

Notes

• Docs-only follow-up after the production Apps Script deploy to version 103.
• No Apps Script push or deploy is included in this change.