feat: tanstack ai middleware

docs/config.json

@@ -50,6 +50,10 @@
           "label": "Agentic Cycle",
           "to": "guides/agentic-cycle"
         },
+        {
+          "label": "Middleware",
+          "to": "guides/middleware"
+        },
         {
           "label": "Structured Outputs",
           "to": "guides/structured-outputs"