Reactivate and curate Accessiblity checker

Revert !607 (merged) or introduce a better one

• Check which software would be appropriate
• Integrate
• Curate / prioritize (only really helpful messages please)

/cc @frust45

assigned to @jandt-desy

added Accessibility label

marked this issue as related to #439 (closed)

mentioned in issue #439 (closed)

I just figured out that the GitLab CI built-in accessibility scanning is deprecated - https://docs.gitlab.com/ee/ci/testing/accessibility_testing.html. So it's definitely time to look for something else.