add csi_plugin_gc_threshold config documentation

website/pages/docs/configuration/server.mdx

@@ -90,6 +90,10 @@ server {
   deployment must be in the terminal state before it is eligible for garbage
   collection. This is specified using a label suffix like "30s" or "1h".
 
+- `csi_plugin_gc_threshold` `(string: "1h")` - Specifies the minimum age of
+  CSI plugin before it is eligible for garbage collection if not in use.
+  This is specified using a label suffix like "30s" or "1h".
+
 - `default_scheduler_config` <code>([scheduler_configuration][update-scheduler-config]:
   nil)</code> - Specifies the initial default scheduler config when
   bootstrapping cluster. The parameter is ignored once the cluster is bootstrapped or