added article to toc

Author: Michael Bender

articles/firewall/toc.yml

@@ -55,6 +55,8 @@ items:
       href: premium-features.md
     - name: Azure Firewall preview features
       href: firewall-preview.md
+    - name: Choose an Azure Firewall
+      href: choose-firewall-sku.md
     - name: IDPS signature rule categories
       href: idps-signature-categories.md
     - name: Azure Firewall Premium in the portal