Update GCP Alertmanager webhook config to use simplified TOKEN format

Simplify the credentials placeholder in the webhook configuration
from explicit Bearer <ACCOUNT_ID> <SIGNING_KEY> to a single <TOKEN>
placeholder with explanatory comment.

Author: claude

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

@@ -36,8 +36,8 @@ Apply the following Secret in the GMP namespace (default ``gmp-public``). Replac
              - url: 'https://api.robusta.dev/integrations/generic/alertmanager'
                http_config:
                  authorization:
-                   # Replace <ACCOUNT_ID> <SIGNING_KEY>, with your account_id and signing_key
-                   credentials: 'Bearer <ACCOUNT_ID> <SIGNING_KEY>'
+                   # Replace <TOKEN> with a string in the format <ACCOUNT_ID> <SIGNING_KEY>
+                   credentials: <TOKEN>
                send_resolved: true
 
        route: