Update Slack embed with improved sizing and security attributes

Upgraded to the new Cloud Posse Slack embed URL with better responsive design, larger dimensions (700px height, full width with 3xl max constraint), and added security sandbox attributes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: osterman

website/docs/community/slack.mdx

@@ -10,15 +10,9 @@ id: slack
 
 Atmos has a great community of active users who are all more than willing to help each other out. So, join us!
 
-<iframe src="https://slack.cloudposse.com/?theme=dark&background=00000000"
-        marginWidth="0"
-        marginHeight="0"
-        hspace="0"
-        vspace="0"
-        width="300"
-        height="330"
-        scrolling="no"
-        frameBorder="0"
-        allowtransparency="true"
-        style={{ borderRadius: '6px', display: 'flex', marginRight: 'auto', marginLeft: 'auto'}}
-        ></iframe>
+<iframe
+  src="https://cloudposse.com/embed/slack?theme=dark&bg=transparent"
+  className="h-[700px] w-full max-w-3xl mx-auto rounded-md border-0"
+  title="Join our Slack Community"
+  sandbox="allow-same-origin allow-scripts allow-forms"
+/>