[0.6 Release] Add 0.6 to ET docs index (#9798)

Author: mergennachin

versions.html

@@ -19,6 +19,9 @@ <h1> ExecuTorch Documentation </h1>
         <li class="toctree-l1">
           <a class="reference internal" href="main/">main (unstable)</a>
         </li>
+        <li class="toctree-l1">
+          <a class="reference internal" href="0.6">0.6 (release candidate)</a>
+        </li>
         <li class="toctree-l1">
           <a class="reference internal" href="0.5">0.5 (stable release)</a>
         </li>