[0.5 Release] Add 0.5 to ET docs index (#7749)

Author: SS-JIA

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="0.5">0.5 (release candidate)</a>
+        </li>
         <li class="toctree-l1">
           <a class="reference internal" href="0.4">0.4 (stable release)</a>
         </li>