Add Learn card to Getting Started page

Author: stevep0z

content/contracts-sui/index.mdx

@@ -12,6 +12,9 @@ import { latestStable } from "./latest-versions.js";
   <Card href={`/contracts-sui/${latestStable}`} title={`Overview (${latestStable})`}>
     Install packages, set up a Move project, and run a first end-to-end integration.
   </Card>
+  <Card href={`/contracts-sui/${latestStable}/learn`} title="Learn">
+    In-depth walkthroughs covering safe arithmetic, rounding, overflow handling, and ownership transfer policies.
+  </Card>
 </Cards>
 
 ## Packages