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

docs/content/features/templates.md

@@ -2,7 +2,6 @@
 title: Tera Templates
 description: Customizable templates without recompilation
 weight: 1
-toc: true
 ---
 
 sukr uses [Tera](https://tera.netlify.app/), a Jinja2-like templating engine. Templates are loaded at runtime, so you can modify them without recompiling sukr.

docs/site.toml

@@ -10,3 +10,4 @@ templates = "templates"
 
 [nav]
 nested = true
+toc    = true