Adding link to Extension documentation.

Author: mikecp

16/umbraco-cms/reference/content-delivery-api/additional-preview-environments-support.md

@@ -7,7 +7,7 @@ description: >-
 
 {% hint style="warning" %}
 The original contents of this article, explaining how to change the backoffice UI, does not apply anymore to Umbraco 15 and above and has been removed.
-The way to achieve this beyond Umbraco 14 is to create and register an extension to modify the backoffice UI preview button.
+The way to achieve this beyond Umbraco 14 is to create and register an [Extension](../../customizing/extending-overview) to modify the backoffice UI preview button.
 {% endhint %}
 
 With Umbraco, you can save and preview draft content before going live. The preview feature allows you to visualize how a page will look like once it is published, directly from within the backoffice. This is also possible for the Content Delivery API data. You can extend the preview functionality in the backoffice by configuring external preview URLs for client libraries consuming the Content Delivery API.