Use Twig pages function for navigation

See https://github.com/picocms/Pico/commit/681ad2715870df3bf5e052bccc34eb9e8f90aaa9 for more info

index.twig

@@ -32,13 +32,10 @@
             </h1>
             <div id="nav" role="region" tabindex="-1">
                 <ul>
-                    {% for page in pages if page.title and not page.hidden %}
-                        {% set pageDepth = page.id|split('/')|length %}
-                        {% if (pageDepth == 2) and (page.id ends with "/index") or (pageDepth == 1) %}
+                    {% for page in pages(depthOffset=-1) if page.title and not page.hidden %}
                         <li{% if page.id == current_page.id %} class="active"{% endif %}>
                             <a href="{{ page.url }}">{{ page.title }}</a>
                         </li>
-                        {% endif %}
                     {% endfor %}
                 </ul>
             </div>