feat(config): add global nav.toc setting with frontmatter override

- Add toc: bool to [nav] config section (default: false)
- Frontmatter toc: true/false overrides global config
- If frontmatter toc is not specified, falls back to config.nav.toc
- Enable nav.toc = true in docs/site.toml for global TOC

src/config.rs

@@ -28,6 +28,9 @@ pub struct SiteConfig {
 pub struct NavConfig {
     /// Whether to display nested navigation (default: false).
     pub nested: bool,
+    /// Enable table of contents (anchor nav) globally (default: false).
+    /// Can be overridden per-page via frontmatter toc field.
+    pub toc: bool,
 }
 
 /// Path configuration with sensible defaults.