Move external dashboard docs

- To user/project/operations
- And cross-link in ci/environments

doc/ci/environments.md

@@ -617,6 +617,10 @@ versions of the app, all without leaving GitLab.
 
 ![Monitoring dashboard](img/environments_monitoring.png)
 
+#### Linking to external dashboard
+
+Add a [button to the Monitoring dashboard](../user/project/operations/linking_to_an_external_dashboard.md) linking directly to your existing external dashboards.
+
 ### Web terminals
 
 > Web terminals were added in GitLab 8.15 and are only available to project Maintainers and Owners.

doc/user/project/operations/index.md

@@ -9,3 +9,4 @@ your applications:
 - Discover and view errors generated by your applications with [Error Tracking](error_tracking.md).
 - Create, toggle, and remove [Feature Flags](feature_flags.md). **[PREMIUM]**
 - [Trace](tracing.md) the performance and health of a deployed application. **[ULTIMATE]**
+- Add a [button to the Monitoring dashboard](linking_to_an_external_dashboard.md) linking directly to your existing external dashboards.

doc/user/project/operations/linking_to_an_external_dashboard.md

@@ -0,0 +1,19 @@
+# Linking to an external dashboard
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/57171) in GitLab 12.0.
+
+You can add a button to the Monitoring dashboard linking directly to your existing external dashboards.
+
+## Enabling the external dashboard link
+
+1. Go to **Settings > Operations** and scroll to the section titled **External dashboard**.
+
+1. Fill in the URL to your external dashboard and click **Save changes**.
+
+   ![External Dashboard Settings](img/external_dashboard_settings.png)
+
+1. There should now be a button on your
+   [Monitoring dashboard](../../../ci/environments.md#monitoring-environments) which
+   will open the URL you entered in the above step.
+
+   ![External Dashboard Link](img/external_dashboard_link.png)