docs: add simple example for IRM incident webhooks (#2291)

Author: matiasb

docs/resources/oncall_outgoing_webhook.md

@@ -29,6 +29,15 @@ resource "grafana_oncall_outgoing_webhook" "test-acc-outgoing_webhook" {
   // Optional: specify the team to which the outgoing webhook belongs
   team_id = data.grafana_oncall_team.my_team.id
 }
+
+resource "grafana_oncall_outgoing_webhook" "test-acc-outgoing_webhook-incident" {
+  provider     = grafana.oncall
+  name         = "my outgoing incident webhook"
+  preset       = "incident_webhook"
+  http_method  = "POST"
+  url          = "https://example.com/"
+  trigger_type = "incident declared"
+}
 ```
 
 <!-- schema generated by tfplugindocs -->

examples/resources/grafana_oncall_outgoing_webhook/resource.tf

@@ -14,3 +14,12 @@ resource "grafana_oncall_outgoing_webhook" "test-acc-outgoing_webhook" {
   // Optional: specify the team to which the outgoing webhook belongs
   team_id = data.grafana_oncall_team.my_team.id
 }
+
+resource "grafana_oncall_outgoing_webhook" "test-acc-outgoing_webhook-incident" {
+  provider     = grafana.oncall
+  name         = "my outgoing incident webhook"
+  preset       = "incident_webhook"
+  http_method  = "POST"
+  url          = "https://example.com/"
+  trigger_type = "incident declared"
+}