Fix: Remove duplicate column tag in What's New page

- Remove duplicate :::column::: opener that created broken column layout
- AI page fixes (ai-build.md, ai-setup.md, ai-for-windows-developers.md) not applied:
  these files don't exist in this repo

Co-authored-by: Copilot <[email protected]>

Author: Qiutong Shen

hub/apps/whats-new/whats-new-for-developers.md

@@ -26,7 +26,6 @@ Find the latest downloads, release notes, and updates for the Windows SDK, Windo
         [View downloads](../windows-app-sdk/downloads.md)
     :::column-end:::
     :::column:::
-        :::column:::
         ![Windows SDK icon](images/wsdk-hero.png)<br>
         **Windows SDK**<br>
         Discover what's new<br>