add a download link in the index file

Author: niccokunzmann

index.html

@@ -14,5 +14,6 @@
         <a href="sv/00-01.html">Svenska</a>
       </li>
     </ul>
+    <a href="https://niccokunzmann.github.io/download_latest/regex-tutorial.zip">Download</a>
   </body>
 </html>