docs: link official doc for plugin installation (#3839)

Author: s4kh

charts/grafana/Chart.yaml

@@ -1,6 +1,6 @@
 apiVersion: v2
 name: grafana
-version: 9.3.1
+version: 9.3.2
 appVersion: 12.1.0
 kubeVersion: "^1.8.0-0"
 description: The leading tool for querying and visualizing time series and metrics.

charts/grafana/README.md

@@ -795,3 +795,6 @@ grafana.ini:
   alerting:
     enabled: false
 ```
+
+### Installing plugins
+For installing plugins please see the [official documentation](https://grafana.com/docs/grafana/latest/administration/plugin-management/#install-plugins-using-the-grafana-helm-chart).