andwati.com/templates/partials/toc.html

{% if page.toc %}
<details class="post-toc">
  <summary>Table of Contents</summary>
  <ul>
    {% for h1 in page.toc %}
    <li>
      <a href="{{ h1.permalink | safe }}">{{ h1.title }}</a>
      {% if h1.children %}
      <ul>
        {% for h2 in h1.children %}
        <li><a href="{{ h2.permalink | safe }}">{{ h2.title }}</a></li>
        {% endfor %}
      </ul>
      {% endif %}
    </li>
    {% endfor %}
  </ul>
</details>
{% endif %}