Add link to 2.3 RC docs (#1590)

Author: svekars

docs/versions.html

@@ -20,6 +20,9 @@ <h1> PyTorch 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="2.3/">v2.3.0 (release candidate)</a>
+        </li>
         <li class="toctree-l1">
           <a class="reference internal" href="2.2/">v2.2.0 (stable)</a>
         </li>