Abacus/docs/_includes/header.html

<nav>
  <div class="center">
    <a href="{{ "/" | relative_url }}" class="primary-link">{{ site.title }}</a>
    {% for page in site.pages %}
    {% if page.in_header %}
    <a href="{{ page.url | relative_url }}">{{ page.title }}</a>
    {% endif %}
    {% endfor %}
  </div>
</nav>