Replaced bad placeholder.md

Author: jmatthiesen

docs/ai/toc.yml

@@ -60,17 +60,17 @@ items:
   - name: Implement RAG using vector search
     href: tutorials/tutorial-ai-vector-search.md
   - name: Loading CSV files
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
   - name: Loading HTML
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
   - name: Loading JSON
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
   - name: Loading Office Documents
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
   - name: Loading PDFs
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
   - name: Loading SQL Data
-    href: placeholder.md
+    href: dotnet-ai-ecosystem.md
 - name: Security & Safety
   items:
   - name: Authenticate App Service to Azure OpenAI