add cdn and afd price compare article entry

Author: duongau

articles/frontdoor/TOC.yml

@@ -236,6 +236,8 @@
       href: front-door-faq.yml
   - name: CDN
     items:
+    - name: CDN and Front Door price comparison
+      href: compare-cdn-front-door-price.md
     - name: Migrate CDN provider
       href: ../cdn/cdn-change-provider.md?toc=/azure/frontdoor/TOC.json
     - name: Akamai retirement FAQ