feat: add ability for EC to define and use Velero plugins

docs/reference/embedded-config.mdx

@@ -265,6 +265,34 @@ spec:
         key: value
 ```
 
+### Configure Velero Plugins
+
+> Introduced in Embedded Cluster v2.13.0
+
+If the customer license has the **Allow Disaster Recovery (Alpha)** option enabled, you can add custom Velero plugins in the `extensions.velero.plugins` key to extend Velero's backup and restore capabilities. For example, you can add support for backing up databases like PostgreSQL, MySQL, or other stateful applications that require advanced disaster recovery capabilities.
+
+Each plugin that you add to the `extensions.velero.plugins` key must have a name and a publicly accessible container image. For more information about the Velero plugin system, including how to create custom plugins, see [Velero plugin system](https://velero.io/docs/v1.17/overview-plugins/) in the Velero documentation.
+
+#### Requirements
+
+* The image for the Velero plugin must be available publicly. Images behind authentication are not supported.
+* The **Allow Disaster Recovery (Alpha)** option must be enabled in the license for Velero plugins to work. For more information about how to enable the Embedded Cluster disaster recovery feature, see [Disaster Recovery for Embedded Cluster (Alpha)](/vendor/embedded-disaster-recovery).
+
+#### Example
+
+```yaml
+apiVersion: embeddedcluster.replicated.com/v1beta1
+kind: Config
+spec:
+  extensions:
+    velero:
+      # Each plugin requires a name and publicly accessible container image
+      plugins:
+        - name: velero-plugin-postgresql
+          image: myvendor/velero-postgresql:v1.0.0
+          imagePullPolicy: Always
+```
+
 ### Configure the Kubelet
 
 You can configure the Kubelet to customize your worker nodes with Embedded Cluster. One common use case for configuring the Kubelet is that you need more pods on a single node than the default limit of 100. In this case, you could set the `maxPods` Kubelet configuration option to 150. Another common example is reducing startup time by setting `maxParallelImagePulls` to increase the maximum number of image pulls that can be done in parallel.

docs/vendor/embedded-disaster-recovery.mdx

@@ -158,7 +158,15 @@ To configure Velero Backup and Restore custom resources for Embedded Cluster dis
        post.hook.restore.velero.io/wait-for-ready: 'true' # waits for the pod to be ready before running the post-restore hook
      ```
 
-1. Save and the promote the release to a development channel for testing.     
+1. Save and the promote the release to a development channel for testing.
+
+### Extend Disaster Recovery with Custom Velero Plugins
+
+> Introduced in Embedded Cluster v2.13.0
+
+You can extend Velero's backup and restore capabilities by configuring custom Velero plugins in the Embedded Cluster Config. This is useful if you need to add support for specific backup scenarios, such as backing up databases like PostgreSQL, MySQL, or other stateful applications that require advanced disaster recovery capabilities.
+
+To configure custom Velero plugins, use the `extensions.velero.plugins` key in the Embedded Cluster Config. For more information, see [Configure Velero Plugins](/reference/embedded-config#configure-velero-plugins) in _Embedded Cluster Config_.
 
 ### Enable the Disaster Recovery Feature for Your Customers