Add UI Quickstart card to quickstart What's Next section

Link to the UI Quickstart from the main quickstart page so users
can easily discover the dashboard-based workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: soumya-io

core/quickstart.mdx

@@ -219,6 +219,10 @@ Key Benefits:
 
 <CardGroup cols={2}>
 
+  <Card title="UI Quickstart" icon="desktop" href="/core/ui-quickstart">
+    Manage agents and controls visually through the dashboard.
+  </Card>
+
   <Card title="Concepts" icon="lightbulb" href="/concepts">
     Learn about controls, selectors, evaluators, and actions.
   </Card>