make the version a link to /versions.html (#3378)

Co-authored-by: Francisco Massa <fvsmassa@gmail.com>

docs/source/_templates/layout.html

+{% extends "!layout.html" %}
+
+{% block sidebartitle %}
+    <div class="version">
+      <a href='http://pytorch.org/docs/vision/versions.html'>{{ version }} &#x25BC</a>
+    </div>
+    {% include "searchbox.html" %}
+{% endblock %}