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/site.toml

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