Add appendix for WebIDL index

Signed-off-by: Zoltan Kis <[email protected]>

Author: zolkis

index.html

@@ -1756,6 +1756,10 @@ <h3>The <dfn data-dfn-for="">Observable</dfn> interface</h3>
       </ul>
   </section>
 
+<section class="appendix" id="idl-index">
+  <!-- ReSpec will gather all Web IDL code here. -->
+</section>
+
   <section> <h2>Acknowledgements</h2>
     <p>
       Special thanks to former editor Johannes Hund (until August 2017, when at Siemens AG) for developing this specification. Also, the editors would like to thank Dave Raggett, Matthias Kovatsch, Michael Koster and Michael McCool for their comments and guidance.