feat(nav): complete Tier 1 hierarchical navigation

Add config option [nav] nested (defaults false), update base.html
template with nested nav rendering, and add .nav-children CSS styling
for indented section children.

- Add NavConfig struct with nested: bool
- Template renders item.children in .nav-children div
- CSS: left border + indent for nested items

docs/templates/base.html

@@ -34,8 +34,17 @@
       </label>
     </header>
     <nav>
-      {% for item in nav %}<a href="{{ prefix }}{{ item.path }}" {% if page_path==item.path %} class="active" {% endif
-        %}>{{ item.label }}</a>{% endfor %}
+      {% for item in nav %}
+      <a href="{{ prefix }}{{ item.path }}" {% if page_path==item.path %}class="active" {% endif %}>{{ item.label }}</a>
+      {% if config.nested_nav and item.children %}
+      <div class="nav-children">
+        {% for child in item.children %}
+        <a href="{{ prefix }}{{ child.path }}" {% if page_path==child.path %}class="active" {% endif %}>{{ child.label
+          }}</a>
+        {% endfor %}
+      </div>
+      {% endif %}
+      {% endfor %}
     </nav>
     <footer class="sidebar-footer">
       <a href="https://github.com/nrdxp/sukr">GitHub</a>