Add accessibility tests

[meduzen]

The automated accessibility tests can run locally or in a GitHub action.

When running in a GitHub action, their results are available as “artifact” (the file attachment in the summary of a GitHub action): the archive contains a HTML file with all the test suites. By opening the accessibility one in a browser and looking into the “attachment”, you get the HTML report dedicated to accessibility issues (as well as a JSON file).

• Things are running.
• WCAG AA violations are in their own test suites instead of being grouped will all other axe-core tags.
• The explainer paragraph is documented at README.md level (or anywhere appropriate in addition to this PR).

[what-the-diff]

PR Summary

• Add accessibility tests
  Enhance product by including accessibility tests to ensure a more inclusive user experience.
• Fix gitignore
  Update the file to ignore test results folder, keeping the working directory clean.
• Update and add dependencies
  Upgrade existing dependencies (sass, axe-html-reporter) and introduce new ones (@axe-core/playwright) for an improved workflow.
• Configure Playwright for HTML reports
  Automatic generation of JSON and HTML accessibility reports in the tests/results directory, easily accessible for both humans and machines. The JSON report is attached to GitHub Actions workflow logs, while the HTML report can be viewed locally or uploaded to external hosting if needed.