site/jekyll/_includes/nav_docs.html

<div class="nav-docs">
  {% for section in site.doc-sections %}
    <h3>{{ section.title }}</h3>
    <ul>
      {% for site_page in site.pages %}
        {% if site_page.url contains section.id | prepend:'/' %}
          <li>
            <a href="{{ site_page.url }}"{% if page.url == site_page.url %} class="active"{% endif %}>
              {{ site_page.title }}
            </a>
          <li>
        {% endif %}
      {% endfor %}
    </ul>
  {% endfor %}
</div>