System hooks doc: token is optional

doc/administration/system_hooks.md

@@ -59,7 +59,8 @@ To create a system hook:
 1. On the left sidebar, at the bottom, select **Admin Area**.
 1. Select **System Hooks**.
 1. Select **Add new webhook**.
-1. Provide the **URL** and **Secret Token**.
+1. Enter the **URL**.
+1. Optional. Enter a [**Secret Token**](../user/project/integrations/webhooks.md#validate-payloads-by-using-a-secret-token).
 1. Select the checkbox next to each optional **Trigger** you want to enable.
 1. Select **Enable SSL verification**, if desired.
 1. Select **Add system hook**.