reference fusion docs (nim-lang#15562)

Author: timotheecour

config/nimdoc.cfg

@@ -132,7 +132,10 @@ doc.body_toc_group = """
         <a href="$theindexhref">Index</a>
       </li>
       <li>
-        <a href="compiler/$theindexhref">Compiler docs</a>
+        <a href="compiler/$theindexhref">compiler docs</a>
+      </li>
+      <li>
+        <a href="https://nim-lang.github.io/fusion/theindex.html">fusion docs</a>
       </li>
     </ul>
   </div>