docs: add deployment runbook for staged releases (refs #15)

Author: Sakeeb91

docs/runbooks/deployment.md

@@ -0,0 +1,28 @@
+# Deployment Runbook
+
+This runbook summarizes the steps operators follow when promoting code through the
+GitHub Actions workflows introduced in issue #15.
+
+## Staging Promotion
+
+1. Merge the feature branch into `main`.
+2. GitHub Actions automatically builds containers and deploys to the staging namespace.
+3. Confirm the workflow step `Run staging smoke tests` succeeds and check the Slack
+   notification for additional context.
+4. Perform any manual QA or database migrations required by the release.
+
+## Production Promotion
+
+1. Create a semver tag (`vX.Y.Z`) on the commit you wish to release.
+2. Approve the production environment gate inside the GitHub Actions run.
+3. Monitor the `Run production smoke tests` step and the accompanying Slack alert.
+4. Record the release in observability tooling (Datadog event logging is automated
+   when credentials are provided).
+
+## Rollback Procedure
+
+1. Re-run the latest successful deployment workflow selecting the previous tag via
+   `workflow_dispatch`.
+2. Alternatively, execute `kubectl rollout undo deployment/<service> -n production`
+   for targeted services.
+3. Update the incident log with root cause, mitigation steps, and follow-up actions.