Only surface prev and next buttons at bottom of page (#6330)

An alternative to #6316 that
removes the previous and next page buttons at the top of each page,
keeping the buttons at the bottom.

This fits the primary purpose of the buttons and eases the transition to
future site designs, which will overhaul this anyway.

Closes #6316
Contribute to #5768

src/_layouts/default.html

@@ -19,7 +19,6 @@
       {% endif -%}
       <article>
         <div class="content">
-          {% include 'navigation-sub.html' -%}
           <div id="site-content-title">
             {% if underscore_breaker_titles -%}
             <h1>{{title | underscoreBreaker}}</h1>
@@ -34,7 +33,7 @@ <h1>{{title }}</h1>
           {% include 'navigation-toc-top.html', tocContents:tocContents -%}
           {% endif -%}
           {{ content }}
-          {% include 'navigation-sub.html' -%}
+          {% render 'navigation-sub.html', prevpage:prevpage, nextpage:nextpage %}
 
           {% include 'page-github-links.html' -%}
         </div>