Fix links to legacy spec documents (#304)

#303 broke the core spec links. This fixes it by adding `core/` to the
directory links and also adds the JS API spec link to the web page.

document/index.html

@@ -69,8 +69,17 @@ <h3>Legacy Extensions</h3>
   <ul>
     <li><p><b>Legacy Exception Handling</b>: defines additional instructions for exception handling that may still be available in some engines and tools, specifically web browsers.</p>
       <ul>
-        <li><a href="legacy/exceptions/">Browser version</a></li>
-        <li><a href="legacy/exceptions/_download/WebAssembly-Legacy-Exceptions.pdf">PDF version</a></li>
+        <li><p><b>Core specification</b></p>
+          <ul>
+            <li><a href="legacy/exceptions/core/">Browser version</a></li>
+            <li><a href="legacy/exceptions/core/_download/WebAssembly-Legacy-Exceptions.pdf">PDF version</a></li>
+          </ul>
+        </li>
+        <li><p><b>JavaScirpt Embedding</b></p>
+          <ul>
+            <li><a href="legacy/exceptions/js-api/">W3C version</a></li>
+          </ul>
+        </li>
       </ul>
     </li>
   </ul>