add new migrate article to TOC

Author: duongau

articles/frontdoor/TOC.yml

@@ -249,6 +249,8 @@
       href: front-door-faq.yml
   - name: CDN
     items:
+    - name: Migrate CDN provider
+      href: ../cdn/cdn-change-provider.md?toc=/azure/frontdoor/TOC.json
     - name: Security baseline
       href: /security/benchmark/azure/baselines/content-delivery-network-security-baseline?toc=/azure/frontdoor/TOC.json
     - name: DDoS protection