Add deprecation notice for webhook payload's secret field (#13330)

docs/content/doc/features/webhooks.en-us.md

@@ -30,6 +30,8 @@ All event pushes are POST requests. The methods currently supported are:
 
 ### Event information
 
+**WARNING**: The `secret` field in the payload is deprecated as of Gitea 1.13.0 and will be removed in 1.14.0: https://github.com/go-gitea/gitea/issues/11755
+
 The following is an example of event information that will be sent by Gitea to
 a Payload URL: