add ms.topic metadata value (MicrosoftDocs#182)

for task 1199273 per Sarah Barrett

Author: v-savila

docs/index.md

@@ -1,5 +1,6 @@
 --- 
 layout: HubPage
+ms.topic: hub-page
 title: Visual C++ Documentation | Microsoft Docs
 description: Learn how to use C++ to develop applications, services, and tools for your platforms and devices.
 ---
@@ -462,4 +463,4 @@ description: Learn how to use C++ to develop applications, services, and tools f
   </li>
   </ul>
  </div>
-</div>
+</div>