Build navigation list include helper

- Update navigation.yml to support multiple navigations and levels
- Optional include navigation into left sidebar

_includes/sidebar.html

@@ -15,6 +15,9 @@
       {% if s.title %}<h3>{{ s.title }}</h3>{% endif %}
       {% if s.text %}{{ s.text | markdownify }}{% endif %}
     {% endfor %}
+    {% if page.sidebar.nav %}
+      {% include nav_list.html items=page.sidebar.nav %}
+    {% endif %}
   {% endif %}
   </div>
 {% endif %}