adding legacy ai pricing page

Author: AarDavMax

articles/azure-monitor/toc.yml

@@ -1665,6 +1665,8 @@ items:
     items:
     - name: View and estimate charges
       href: usage-estimated-costs.md
+    - name: Legacy Application Insights pricing
+      href: https://learn.microsoft.com/previous-versions/azure/azure-monitor/app/legacy-pricing
     - name: Azure Monitor Pricing page
       href: https://azure.microsoft.com/pricing/details/monitor/
     - name: Pricing calculator