Increase website accessability

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Right now the website is really unusable when using keyboard navigation or screenreaders.

ToDo:

• Alt-Tags for all images
• Make more use of semantic HTML
• Fix focus order/keyboard navigation
• Add focus indicator
• Check screenreader usability
• Review and fix mobile view
• Improve color contrast
  • Color contrast of navbar
  • ...?

General guidelines:
https://www.w3.org/WAI/WCAG21/quickref/?versions=2.0

[unfokus]

Guideline for expandable menu bar accessibility: https://www.w3.org/WAI/tutorials/menus/flyout/#flyoutnavkbbtn

[unfokus]

Color Contrast Checker: https://colourcontrast.cc/