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/site.toml

@@ -7,3 +7,6 @@ content   = "content"
 output    = "public"
 static    = "static"
 templates = "templates"
+
+[nav]
+nested = true