Update GMP Prometheus webhook config to use external API with http_config

Change webhook URL from internal cluster endpoint to external
api.robusta.dev endpoint with proper http_config authorization.

Author: claude

docs/configuration/alertmanager-integration/google-managed-prometheus.rst

@@ -34,7 +34,11 @@ To send alerts to Robusta, create an AlertManager configuration file with the na
    receivers:
      - name: 'robusta'
        webhook_configs:
-         - url: 'http://<helm-release-name>-runner.<namespace>.svc.cluster.local/api/alerts'
+         - url: 'https://api.robusta.dev/integrations/generic/alertmanager'
+           http_config:
+             authorization:
+               # Replace <TOKEN> with a string in the format <ACCOUNT_ID> <SIGNING_KEY>
+               credentials: <TOKEN>
            send_resolved: true
      - name: 'default-receiver'