A11Y-Improvements

[therobrob]

We've taken accessibility into account from the very beginning. However, some things need to be reviewed and improved:

• Check and standardize focus indicators
• Check heading hierarchy
• Check usability with screen reader (at best NVDA & VoiceOver)
• Is the <hr> element captured by the screen reader? If so, we should hide it
• Can we improve the SR-support in the markdown-pages? e. g.
  --> does role="doc-endnotes" need title / aria-label / aria-labelledby?
  --> The marks of ol-elements are currently read by SR
• Within the expander, there are <br> that are read as "Empty Object." Can we remove them?
• aria-label / aria-hidden for images
• teaser hover & focus state, discussed in feat: Implement enhancements for new design #192 (comment) (implementation like bahnhof.de)