docs-app: restore site styling layer, add page-walking a11y tests - #22

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4, felixmosh/turborepo-gh-artifacts@v3, pnpm/action-setup@v4.1.0. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/