hacks-guide-minimal-mistakes-theme/_includes/nav_list.html

{% include base_path %}
{% assign navigation = site.data.navigation[page.sidebar.nav] %}

<nav>
  <ul>
    {% for nav in navigation %}
      <li>{{ nav.title }}
        {% if nav.children != null %}
        <ul>
          {% for child in nav.children %}
            {% assign nav_url = child.path | prepend: "/" | prepend: page.sidebar.nav | prepend: "/" | append: "/" | prepend: base_path %}
            {% if nav_url contains page.url %}
              {% assign c = "current" %}
            {% else %}
              {% assign c = "" %}
            {% endif %}
            <li><a href="{{ nav_url }}" class="{{ c }}">{{ child.title }}</a></li>
          {% endfor %}
        </ul>
        {% endif %}
      </li>
    {% endfor %}
  </ul>
</nav>