Add PostgreSQL HA cluster guide

source/administration-guide/scale/high-availability-cluster-based-deployment.rst

@@ -30,7 +30,9 @@ Moreover, search replicas are also supported to handle search queries.
 Deployment guide
 ----------------
 
-Set up and maintain a high availability cluster-based deployment on your Mattermost servers. This document doesn't cover the configuration of databases in terms of disaster recovery, however, you can refer to the `frequently asked questions (FAQ)`_ section for our recommendations.
+Set up and maintain a high availability cluster-based deployment on your Mattermost servers. Database-level HA and disaster recovery design are out of scope for this document.
+For self-hosted deployments requiring database-level HA,
+see :doc:`PostgreSQL high availability cluster </administration-guide/scale/postgres-ha-cluster>`.
 
 To ensure your instance and configuration are compatible with a high availability cluster-based deployment, please review the `configuration and compatibility`_ section.