add retain edge domain migration entry

Author: duongau

articles/frontdoor/TOC.yml

@@ -459,7 +459,11 @@
     - name: Edgio
       items:
       - name: Migrate to Azure Front Door
-        href: migrate-cdn-to-front-door.md
+        items:
+        - name: Custom domain
+          href: migrate-cdn-to-front-door.md
+        - name: Retain *.azureedge.net domain
+          href: ../cdn/migrate-cdn-to-front-door-retain-edge-domain.md?toc=/azure/frontdoor/TOC.json
       - name: Standard
         items:
         - name: Optimize content