site/_includes/docs_contents_mobile.html

<div class="docs-nav-mobile unit whole show-on-mobiles">
  <select onchange="if (this.value) window.location.href=this.value">
    <option value="">Navigate the docs…</option>
    {% for section in site.data.docs %}
    <optgroup label="{{ section.title }}">
      {% include docs_option.html items=section.docs %}
    </optgroup>
    {% endfor %}
  </select>
</div>