Add link to the .Net API docuemntation

Author: markwallace-microsoft

agent-framework/TOC.yml

@@ -19,4 +19,5 @@ items:
 - name: Migration Guide
   href: migration-guide/TOC.yml
 - name: API Reference Guide
-  items:
+  href: api-docs/TOC.yml
+  expanded: true

agent-framework/api-docs/TOC.yml

@@ -0,0 +1,4 @@
+- name: C# API reference
+  href: /dotnet/api/microsoft.agents.ai
+- name: Python API reference
+  href: /python/api/agent-framework