site/_includes/section_nav.html

{% comment %}
Map grabs the doc sections, giving us an array of arrays. Join, flattens all
the items to a comma delimited string. Split turns it into an array again.
{% endcomment %}
{% assign docs = site.data.docs | map: 'docs' | join: ',' | split: ',' %}

{% comment %}
Because this is built for every page, lets find where we are in the ordered
document list by comparing url strings. Then if there's something previous or
next, lets build a link to it.
{% endcomment %}

{% for document in docs %}
  {% assign document_url = document | prepend:"/docs/" | append:".html" %}
  {% if document_url == page.url %}
    <div class="section-nav">
      <div class="left align-right">
          {% if forloop.first %}
            <span class="prev disabled">Back</span>
          {% else %}
            {% assign previous = forloop.index0 | minus: 1 %}
            {% assign previous_page = docs[previous] | prepend:"/docs/" | append:".html" %}
            <a href="{{ previous_page }}" class="prev">Back</a>
          {% endif %}
      </div>
      <div class="right align-left">
          {% if forloop.last %}
            <span class="next disabled">Next</span>
          {% else %}
            {% assign next = forloop.index0 | plus: 1 %}
            {% assign next_page = docs[next] | prepend:"/docs/" | append:".html" %}
            <a href="{{ next_page }}" class="next">Next</a>
          {% endif %}
      </div>
    </div>
    <div class="clear"></div>
    {% break %}
  {% endif %}
{% endfor %}