feat(nav): add anchor TOC in sidebar for toc-enabled pages

- Pass extracted anchors through template context
- Render anchor links under active page when page.toc is true
- Hierarchical indentation by heading level (h2-h6)
- CSS styling: smaller font, muted color, border-left indicator
- Add toc: true to templates.md as example

docs/content/features/templates.md

@@ -2,6 +2,7 @@
 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.