Add custom tag for referencing regions as per https://review.learn.microsoft.com/en-us/help/contribute/contribute-get-started-channel-guidance?branch=main#content-that-does-not-belong-in-a-docsmicrosoftcom-article-or-repository since this is public preview

Author: tomkerkhove

articles/api-management/includes/preview/preview-callout-self-hosted-gateway-azure-arc.md

@@ -4,6 +4,7 @@ ms.topic: include
 author: dlepow
 ms.author: danlep
 ms.date: 05/25/2021
+ms.custom: references_regions
 ---
 
 > [!IMPORTANT]