added toc entry

Author: Justinha

articles/active-directory/authentication/TOC.yml

@@ -84,6 +84,8 @@
     href: concept-registration-mfa-sspr-combined.md
   - name: Resilient access controls
     href: concept-resilient-controls.md
+  - name: Web browser cookies
+    href: concept-authentication-web-browser-cookies.md
 - name: How-to guides
   items:
   - name: Manage authentication methods