doc(hugo): Add hugo upgrade process

docs/hugo-upgrade.md

@@ -0,0 +1,27 @@
+# Hugo upgrade
+
+[Hugo](https://github.com/gohugoio/hugo) engine is frequently updated by the team to add features or bug fixes.
+
+Here is the process to upgrade Hugo:
+
+1. Test new Hugo version locally
+
+    1. Launch Hugo with new version:
+
+        ```bash
+        export HUGO_VERSION=<new_version>
+        docker compose up
+        ```
+
+    1. Check website rendering on <http://localhost:1313>
+
+        Checklist:
+
+        - Check runbook (including SQL rendering) in runbook (e.g. `PostgreSQLInactiveLogicalReplicationSlot` runbook)
+        - Check tabs are correctly rendered in tutorials (e.g. `RDS exporter deployment`)
+        - Check search engine
+        - Check "last update" information in page footer
+
+1. Upgrade Hugo version for local development environment in `docker-compose.yaml`
+
+1. Upgrade Hugo version for CI in `.github/workflows/release.yaml`