add retirement faq to TOC

Author: duongau

articles/frontdoor/TOC.yml

@@ -174,6 +174,7 @@
             href: tier-migration.md
           - name: Classic to Standard/Premium mapping
             href: tier-mapping.md
+        - name: Front Door (classic) retirement FAQ
         - name: Caching
           href: front-door-caching.md?pivots=front-door-classic
         - name: Traffic acceleration