docs(kuma-cp): add short explanation of high availability

app/_data/docs_nav_kuma_2.9.x.yml

@@ -66,6 +66,8 @@ items:
             url: /production/deployment/single-zone/
           - text: Multi-zone deployment
             url: /production/deployment/multi-zone/
+          - text: High availability
+            url: /production/deployment/high-availability/
       - text: Use Kuma
         url: /production/use-mesh/
       - title: Control plane deployment

app/_src/production/deployment/high-availability.md

@@ -0,0 +1,30 @@
+---
+title: High availability
+content_type: explanation
+---
+
+In order to ensure high availability (HA) of a control plane, both global and zone,
+you can deploy more than one replica.
+
+Each replica should be behind a load balancer such that requests to the control
+plane API are distributed across the replicas.
+
+On Kubernetes this is handled using a Deployment and a Service.
+When your control plane isn't deployed on Kubernetes, it's up to you to handle
+this within your infrastructure.
+
+## Leader
+
+Control planes also perform tasks independent of API requests. To avoid
+conflicts, each of these tasks needs to be performed by only one instance at a
+time, by the _leader_.
+
+For each logical control plane, the instances work together to ensure there's only one
+leader at a time. When the control planes are running in universal mode, this is
+handled by the leader maintaining a lock on the database. In Kubernetes mode, it's
+handled similarly but uses the built-in coordination API resources.
+
+An example of a leader task is that a zone control plane leader
+maintains a connection to the global control plane and both sends relevant
+changes in the zone to the global control plane and receives relevant updates
+about the global state.