mkdocs-material/material/partials/toc-item.html

<li class="md-nav__item">
  <a href="{{ toc_item.url }}" title="{{ toc_item.title }}" class="md-nav__link">
    {{ toc_item.title }}
  </a>
  {% if toc_item.children %}
    <nav class="md-nav">
      <ul class="md-nav__list">
        {% for toc_item in toc_item.children %}
          {% include "partials/toc-item.html" %}
        {% endfor %}
      </ul>
    </nav>
  {% endif %}
</li>