Updated site navigation generation to simplify prev/next etc.

Now the page order is controlled by _data/docs.yml only making it simple to add documents and edit their order.

_includes/section_nav.html

@@ -1,22 +1,39 @@
-<div class="section-nav">
-  <div class="left align-right">
-    {% if page.prev_section != null %}
-      <a href="{{ site.baseurl }}/docs/{{ page.prev_section }}/" class="prev">
-        Back
-      </a>
-    {% else %}
-      <span class="prev disabled">Back</span>
-    {% endif %}
-  </div>
-  <div class="right align-left">
-    {% if page.next_section != null %}
-      <a href="{{ site.baseurl }}/docs/{{ page.next_section }}/" class="next">
-        Next
-      </a>
-    {% else %}
-      <span class="next disabled">Next</span>
-    {% endif %}
-  </div>
-  <div class="clear"></div>
-</div>
+{% 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:"/" %}
+  {% 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:"/" %}
+            <a href="{{ site.baseurl }}{{ 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:"/" %}
+            <a href="{{ site.baseurl }}{{ next_page }}" class="next">Next</a>
+          {% endif %}
+      </div>
+    </div>
+    <div class="clear"></div>
+    {% break %}
+  {% endif %}
+{% endfor %}