sergio.am/_includes/navigation.html

<nav class="post-nav">
  {% if page.previous %}
  <a
    class="post-nav-item post-nav-prev"
    href="{{ page.previous | relative_url }}"
  >
    <div class="nav-arrow">↶ Anterior</div>
    <span class="post-title">{{ page.previous.title }}</span>
  </a>
  {% endif %} {% if page.next %}
  <a class="post-nav-item post-nav-next" href="{{ page.next | relative_url }}">
    <div class="nav-arrow">Siguiente ↷</div>
    <span class="post-title">{{ page.next.title }}</span>
  </a>
  {% endif %}
</nav>