Utilities/Sphinx: Add option for cmake.org version switch to html

Leave the option undocumented since it is not for general use.  We can
simply pass it through `SPHINX_FLAGS` when building for publication on
`cmake.org`.

Utilities/Sphinx/templates/layout.html

@@ -8,10 +8,22 @@
     <a href="https://cmake.org/">CMake</a>{{ reldelim1 }}
   </li>
   <li>
+    {%- if versionswitch is defined %}
+    <span class="version_switch">{{ release }}</span>
+    <a href="{{ pathto(master_doc) }}">{% trans %}Documentation{% endtrans %}</a>{{ reldelim1 }}
+    {%- else %}
     <a href="{{ pathto(master_doc) }}">{{ shorttitle|e }}</a>{{ reldelim1 }}
+    {%- endif %}
   </li>
 {% endblock %}
 
+{% block extrahead %}
+  {% if versionswitch is defined %}
+  <script type="text/javascript" src="{{ pathto('../version_switch.js', 1) }}"></script>
+  {% endif %}
+{{ super() }}
+{% endblock %}
+
 {# Put some context in the html title element. Workaround for #}
 {# https://bitbucket.org/birkenfeld/sphinx/issue/1492/qthelp-generate-html-title-element-should #}
 {% block htmltitle %}