Administration/Clustering: Add "Expand Cluster" page

... linking to community forum article "How to add new nodes to an
existing cluster".

-- https://community.cratedb.com/t/how-to-add-new-nodes-to-an-existing-cluster/1546

docs/admin/clustering/expand.md

@@ -0,0 +1,21 @@
+(expand-cluster)=
+
+# Expand Cluster
+
+Learn how to add new nodes to an existing on-premises database cluster.
+
+A significant feature in CrateDB is that it can scale horizontally, which means
+that instead of adding more RAM, CPU, and disk resources to existing nodes, you
+will add more individual nodes to your CrateDB cluster.
+
+By running your database cluster on multiple nodes, you will gain two benefits.
+
+- Store data volumes larger than being able to be handled on a single machine.
+- Add resiliency to your distributed database cluster, by increasing the number
+  of replica nodes.
+
+The article about [how to add new nodes to an existing cluster] walks you
+through the process of scaling up your database cluster.
+
+
+[how to add new nodes to an existing cluster]: https://community.cratedb.com/t/how-to-add-new-nodes-to-an-existing-cluster/1546

docs/admin/clustering/index.rst

@@ -14,6 +14,7 @@ adding more nodes. This section of the documentation covers that topic.
 
    multi-node-setup
    multi-zone-setup
+   expand
    scale-up-down
    kubernetes
    logical-replication-setup