Add YouTube video iframe to Slack integration documentation

Author: openhands-agent

docs/usage/cloud/slack-installation.mdx

@@ -3,6 +3,16 @@ title: Slack Integration (Beta)
 description: This guide walks you through installing the OpenHands Slack app.
 ---
 
+<iframe
+  width="560"
+  height="315"
+  src="https://www.youtube.com/embed/hbloGmfZsJ4"
+  title="OpenHands Slack Integration Tutorial"
+  frameBorder="0"
+  allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
+  allowFullScreen>
+</iframe>
+
 ## Prerequisites
 
 - Access to OpenHands Cloud.