Alerting: Add external Alertmanagers (#39183)

* building ui

* saving alertmanager urls

* add actions and api call to get external ams

* add list to add modal

* add validation and edit/delete

* work on merging results

* merging results

* get color for status heart

* adding tests

* tests added

* rename

* add pollin and status

* fix list sync

* fix polling

* add info icon with actual tooltip

* fix test

* Accessibility things

* fix strict error

* delete public/dist files

* Add API tests for invalid URL

* start redo admin test

* Fix for empty configuration and test

* remove admin test

* text updates after review

* suppress appevent error

* fix tests

* update description

Co-authored-by: gotjosh <josue@grafana.com>

* fix text plus go lint

* updates after pr review

* Adding docs

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* Update docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

Co-authored-by: gotjosh <josue@grafana.com>

* prettier

* updates after docs feedback

Co-authored-by: gotjosh <josue.abreu@gmail.com>
Co-authored-by: gotjosh <josue@grafana.com>

docs/sources/alerting/unified-alerting/fundamentals/alertmanager.md

@@ -15,3 +15,25 @@ Grafana includes built-in support for Prometheus Alertmanager. By default, notif
 Grafana 8 alerting added support for external Alertmanager configuration. When you add an [Alertmanager data source]({{< relref "../../../datasources/alertmanager.md" >}}), the Alertmanager drop-down shows a list of available external Alertmanager data sources. Select a data source to create and manage alerting for standalone Cortex or Loki data sources.
 
 {{< figure max-width="40%" src="/static/img/docs/alerting/unified/contact-points-select-am-8-0.gif" max-width="250px" caption="Select Alertmanager" >}}
+
+You can configure one or several external Alertmanagers to receive alerts from Grafana. Once configured, both the embedded Alertmanager **and** any configured external Alertmanagers will receive _all_ alerts.
+
+You can do the setup in the "Admin" tab within the Grafana v8 Alerts UI.
+
+### Add a new external Alertmanager
+
+1. In the Grafana menu, click the Alerting (bell) icon to open the Alerting page listing existing alerts.
+2. Click **Admin** and then scroll down to the External Alertmanager section.
+3. Click **Add Alertmanager** and a modal opens.
+4. Add the URL and the port for the external Alertmanager. You do not need to specify the path suffix, for example, `/api/v(1|2)/alerts`. Grafana automatically adds this.
+
+The external URL is listed in the table with a pending status. Once Grafana verifies that the Alertmanager is discovered, the status changes to active. No requests are made to the external Alertmanager at this point; the verification signals that alerts are ready to be sent.
+
+### Edit an external Alertmanager
+
+1. Click the pen symbol to the right of the Alertmanager row in the table.
+2. When the edit modal opens, you can view all the URLs that were added.
+
+The edited URL will be pending until Grafana verifies it again.
+
+{{< figure max-width="40%" src="/static/img/docs/alerting/unified/ext-alertmanager-active.png" max-width="650px" caption="External Alertmanagers" >}}