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 %}{% comment %}
{% endcomment %}    <optgroup label="{{ section.title }}">{% comment %}
{% endcomment %}      {% include docs_option.html items=section.docs %}{% comment %}
{% endcomment %}    </optgroup>{% comment %}
{% endcomment %}
{% endfor %}
  </select>
</div>