liquid/_includes/sidebar.html

<div class="sidebar">
  <div class="sidebar--logo">
    <a href="/">Liquid</a>
  </div>
  <nav class="sidebar--nav">

    {% capture folders %}basics,tags,filters{% endcapture %}
    {% assign sections = folders | split: "," %}

    {% for section in sections %}
    <ul class="section">
      <li>
        <h3 class="section__header">{{ section | capitalize }}</h3>
        <ul class="section__links">
          {% for page in site.pages %}
            {% if page.path contains section/ %}
              {% unless page.path contains "index.html" %}
              <li><a href="{{ page.url | prepend: site.baseurl }}" class="section__link">{{ page.title }}</a></li>
              {% endunless %}
            {% endif %}
          {% endfor %}
        </ul>
      </li>
    </ul>
    {% endfor %}

  </nav>
</div>