docs(alerting): add JSON payload example to template examples list (#104282)

docs/sources/shared/alerts/example-custom-json-payload.md

@@ -0,0 +1,44 @@
+---
+labels:
+  products:
+    - cloud
+    - enterprise
+    - oss
+title: 'Custom webhook example'
+---
+
+```go
+{{ define "webhook.custom.payload" -}}
+  {{ coll.Dict
+  "receiver" .Receiver
+  "status" .Status
+  "alerts" (tmpl.Exec "webhook.custom.simple_alerts" .Alerts | data.JSON)
+  "groupLabels" .GroupLabels
+  "commonLabels" .CommonLabels
+  "commonAnnotations" .CommonAnnotations
+  "externalURL" .ExternalURL
+  "version" "1"
+  "orgId"  (index .Alerts 0).OrgID
+  "truncatedAlerts"  .TruncatedAlerts
+  "groupKey" .GroupKey
+  "state"  (tmpl.Inline "{{ if eq .Status \"resolved\" }}ok{{ else }}alerting{{ end }}" . )
+  "allVariables"  .Vars
+  "title" (tmpl.Exec "default.title" . )
+  "message" (tmpl.Exec "default.message" . )
+  | data.ToJSONPretty " "}}
+{{- end }}
+
+{{- /* Embed json templates in other json templates. */ -}}
+{{ define "webhook.custom.simple_alerts" -}}
+  {{- $alerts := coll.Slice -}}
+  {{- range . -}}
+    {{ $alerts = coll.Append (coll.Dict
+    "status" .Status
+    "labels" .Labels
+    "startsAt" .StartsAt
+    "endsAt" .EndsAt
+    ) $alerts}}
+  {{- end -}}
+  {{- $alerts | data.ToJSON -}}
+{{- end }}
+```