Refine TOC validation logic and error messaging.

[Mpdreamz]

Improve handling of TOC nesting rules by adding checks for valid file paths and better accommodating development and narrative docs. Update error messages for clarity and adjust depth validation to reflect maximum allowed levels.

(cherry picked from commit 3e61bec)

Relates: #780 (comment)

This ensures that if a toc: section links to other toc sections

• it may at most link to one index.md at the root.
• it must link to one index.md for nested toc.yml files.