_includes/docs_nav.html

<div class="panel-group" id="accordion" role="tablist" aria-multiselectable="true">
  {% for section in site.data.docs %}
    <div class="panel panel-default">
      <div class="panel-heading">
        <h4 class="panel-title">
          <a role="button" data-toggle="collapse" data-parent="#accordion" href="#collapse-{{forloop.index}}" aria-expanded="false" aria-controls="collapse-{{forloop.index}}">
            {{ section.title }}
          </a>
        </h4>
      </div>
      {% assign collapsed = true %}
      {% for item in section.docs %}
        {% assign item_url = item | prepend:"/docs/" | append:"/" %}
        {% if item_url == page.url %}
          {% assign collapsed = false %}
        {% endif %}
      {% endfor %}
      <div class="panel-collapse{% if collapsed %} collapse{% endif %}" id="collapse-{{forloop.index}}" role="tabpanel" aria-label="Side Navigation">
        <div class="list-group">
          {% for item in section.docs %}
            {% assign item_url = item | prepend:"/docs/" | append:"/" %}
            {% assign p = site.docs | where:"url", item_url | first %}
            <a class="list-group-item {% if item_url == page.url %}active{% endif %}" href="{{ p.url  | relative_url }}">{{ p.title }}</a>
          {% endfor %}
        </div>
      </div>
    </div>
  {% endfor %}
</div>