[CI/a11y] Add automated heading-hierarchy / axe-core check to site_lint.yml

Currently, accessibility regressions related to heading structure (such as skipping heading levels or incorrect hierarchy order) are not caught automatically by CI during PR checks.

Recently, a heading level skip issue was identified and resolved manually ([link to your PR here]). Integrating an automated accessibility auditor into our CI pipeline would prevent similar regressions from slipping through in the future.

Proposed Solution

• Add an automated accessibility testing tool (such as axe-core, @axe-core/playwright, or an equivalent linting rule/script) to the site_lint.yml GitHub Actions workflow.
• Ensure the check specifically validates heading-order hierarchy rules (e.g., preventing skips like H1 → H3).

Additional Context

• Discussed as a side note in PR https://github.com/reactjs/react.dev/pull/8607#pullrequestreview-4955873812.

• Tooling suggestion: axe-core / eslint-plugin-jsx-a11y or a post-build HTML scan step in CI.

まず GitHub Actions の workflow である site_lint.yml と、PR 8607 の見出し順序に関する議論を読みます。axe-core、@axe-core/playwright、eslint-plugin-jsx-a11y、または post-build HTML check が既存の workflow にどのように適合するかを判断し、そのうえで CI が H1 → H3 のように見出しレベルがスキップされているケースを検出することを確認します。