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

src/template_engine.rs

@@ -121,6 +121,8 @@ pub struct ConfigContext {
     pub title: String,
     pub author: String,
     pub base_url: String,
+    /// Whether to display nested navigation
+    pub nested_nav: bool,
 }
 
 impl From<&SiteConfig> for ConfigContext {
@@ -129,6 +131,7 @@ impl From<&SiteConfig> for ConfigContext {
             title: config.title.clone(),
             author: config.author.clone(),
             base_url: config.base_url.clone(),
+            nested_nav: config.nav.nested,
         }
     }
 }