Add aria-level to toctree caption headings

[Mirochill]

Summary

• add an explicit aria-level=2 to toctree caption headings rendered with role=heading
• keep the historical implicit heading level explicit so HTML validators accept the markup without changing the exposed outline
• update the existing HTML expectation for translated toctree captions

Rationale

Sphinx has rendered toctree captions as <p role=heading> since #9358. WAI-ARIA requires heading roles to expose aria-level, and ARIA 1.1 defines the omitted value as level 2; making that level explicit preserves the existing semantics while resolving the validator error reported in #14030.

Fixes #14030.

Validation

• Not run locally
• relying on the repository's GitHub Actions / Read the Docs checks for remote validation