[Accessibility] Fix content hierarchy

[thilobillerbeck]

Unfortunately some content hierarchy seems to be broken on different pages.
The goal of this issue is to to check all pages and fix the content hierarchy to make pages more easily navigatable for different reading modes like text-only browsers and screenreaders.

Some reference:
Tags and their types: https://developer.mozilla.org/en-US/docs/Web/HTML/Content_categories
Extension to see content hierarchy at a glance and which also marks errors: https://wave.webaim.org/extension/
Guidelines:

• https://www.w3.org/WAI/tutorials/page-structure/content/
• https://www.w3.org/WAI/tutorials/page-structure/headings/
• https://web.dev/learn/accessibility/structure?hl=de