add href for classic retirement

Author: duongau

articles/frontdoor/TOC.yml

@@ -175,6 +175,7 @@
           - name: Classic to Standard/Premium mapping
             href: tier-mapping.md
         - name: Front Door (classic) retirement FAQ
+          href: classic-retirement-faq.md
         - name: Caching
           href: front-door-caching.md?pivots=front-door-classic
         - name: Traffic acceleration