_includes/docnav.html

<div class="doc-nav">
  {% comment %}The variable already exists through the breadcrumb include: 
  {% assign doc_data = 'docs_' | append: site.active_lang %}
  {% assign page_data = site.data[doc_data].by_title[page.title] %}
  {%endcomment %}
  <span class="previous-toc">{% if page_data.previous_url != '' %}<a href="{{ site.baseurl }}{{ page_data.previous_url }}">← {{ page_data.previous_title }}</a>{% endif %}</span><span class="next-toc">{% if page_data.next_url != '' %}<a href="{{ site.baseurl }}{{ page_data.next_url }}">{{ page_data.next_title }} →</a>{% endif %}</span>
</div>