Doc: add DeepWiki Badge to README.md

Add the DeepWiki Badge code to the README.md file to make the wiki
auto refresh weekly. It provides the user and developer a quick
portal to learn about Cloudberry with AI.

Cloudberry@DeepWiki: https://deepwiki.com/apache/cloudberry

Author: tuhaihe

README.md

@@ -26,6 +26,7 @@
 <a href="https://sonarcloud.io/summary/new_code?id=apache_cloudberry">
   <img alt="SonarQube Cloud" src="https://sonarcloud.io/images/project_badges/sonarcloud-highlight.svg" width="100px">
 </a>
+[![Ask DeepWiki](https://deepwiki.com/badge.svg)](https://deepwiki.com/apache/cloudberry)
 ---------
 
 ## Introduction