added cross link to azure firewall TOC for NAT gateway article

Author: asudbring

articles/firewall/toc.yml

@@ -38,6 +38,8 @@ items:
     href: tutorial-firewall-dnat-policy.md
   - name: Deploy firewall with DDoS protection
     href: tutorial-protect-firewall.md
+  - name: Integrate NAT gateway with Azure Firewall
+    href: /azure/virtual-network/nat-gateway/tutorial-hub-spoke-nat-firewall?toc=/azure/firewall/toc.json
 
 - name: Samples
   items: