Add checking the github oauth app in the troubleshooting page

Ref #6824 (comment)

Author: stsewd

docs/webhooks.rst

@@ -170,6 +170,16 @@ offer a way to check this.
 Troubleshooting
 ---------------
 
+Webhook activation failed. Make sure you have the necessary permissions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you find this error,
+make sure your user has permissions over the repository.
+In case of GitHub,
+check that you have granted access to the Read the Docs `OAuth App`_ to your organization.
+
+.. _OAuth App: https://github.com/settings/applications
+
 My project isn't automatically building
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~