alerting: add incident.io (#1456)

For sourcegraph/sourcegraph#7994

Author: bobheadxi

docs/admin/observability/alerting.mdx

@@ -101,6 +101,23 @@ Once configured, Sourcegraph alerts will automatically be routed to the appropri
 ]
 ```
 
+#### Incident.io
+
+```json
+"observability.alerts": [
+  {
+    "level": "critical",
+    "notifier": {
+      "type": "incidentio",
+      // The incident.io Alert Source URL
+      "url": "https://api.incident.io/v1/alert_sources/...",
+      // The incident.io Alert Source Token. This token can also be set by the environment variable INCIDENT_IO_ALERT_SOURCE_TOKEN on the prometheus container. Setting here takes precedence over the environment variable.
+      "alertSourceToken": "xxxxxx"
+    }
+  }
+]
+```
+
 #### Webhook
 
 ```json