Consider replacing `toc_root` feature by section-as-root

[chalin]

Note that #2328 (section-as-root) might be able to replace the toc_root feature.

• [ ] Ensure to document how this feature can be used in docs-only sites, even without toc_root.
• [ ] See https://www.docsy.dev/docs/adding-content/content/#alternative-site-structure