Add warning about it being the 3rd edition sandbox to /code

Author: marijnh

code/index.html

@@ -17,6 +17,13 @@
   </style>
 </head>
 
+<div style="background: #ffb; border-bottom: 1px solid silver; padding: 6px 4em; text-align: center">
+  <strong>Note:</strong> If you are reading the <strong>second edition</strong> of the book,
+  you'll want to go
+  to <a href="https://eloquentjavascript.net/2nd_edition/code/">that
+  edition's sandbox</a> instead!
+</div>
+
 <article style="max-width: 50em;">
   <h1>Code Sandbox<div style="font-size: 70%"><a class="subtlelink" href="../index.html">Eloquent JavaScript</a></div></h2>