[v9.2.x] Docs: Add feature flag example to override configuration with environment variables (#58615)

Co-authored-by: Alexa V <239999+axelavargas@users.noreply.github.com>

docs/sources/setup-grafana/configure-grafana/_index.md

@@ -71,6 +71,9 @@ client_secret = 0ldS3cretKey
 
 [plugin.grafana-image-renderer]
 rendering_ignore_https_errors = true
+
+[feature_toggles]
+enable = newNavigation
 ```
 
 You can override them on Linux machines with:
@@ -80,6 +83,7 @@ export GF_DEFAULT_INSTANCE_NAME=my-instance
 export GF_SECURITY_ADMIN_USER=owner
 export GF_AUTH_GOOGLE_CLIENT_SECRET=newS3cretKey
 export GF_PLUGIN_GRAFANA_IMAGE_RENDERER_RENDERING_IGNORE_HTTPS_ERRORS=true
+export GF_FEATURE_TOGGLES_ENABLE=newNavigation
 ```
 
 ## Variable expansion