test: add sa11y to showcases to find some a11y issues

[nmerget]

Proposed changes

part of #2579

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (fix on existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

[github-actions]

🔭🐙🐈 Test this branch here: https://db-ui.github.io/mono/review/test-sa11y

[mfranzke]

what's your impression, could this add any value besides what we already have by axe? Did you have any reportings that mentioned further aspects?

[nmerget]

Yes, there are some minor findings for the showcases. I guess it's working better for a "real" app with more content, but besides this we have some additional settings like the color-contrast (which isn't working at the moment because of an issue with color-mix).