add javascript variant feature flag doc

Author: zhiyuanliang-ms

articles/azure-app-configuration/TOC.yml

@@ -208,6 +208,8 @@
       href: howto-variant-feature-flags-aspnet-core.md
     - name: Python
       href: howto-variant-feature-flags-python.md
+    - name: JavaScript
+      href: howto-variant-feature-flags-javascript.md
 - name: CI/CD integration
   items:
   - name: Use configuration files