Add more E2E tests

[tumidi]

• custom content
  • [ ] nav/footer links
  • [ ] footers
  • [ ] branding
• content
  • [ ] LaTeX formular rendering
  • [ ] display of sub-sections
  • [ ] breadcrumb
  • [ ] section/page links
• exercises
  • [ ] rendering
  • [ ] validation
  • [ ] hints
• [x] TOC next/prev navigation
• [ ] search
• media query
  • [x] nav: mobile menu button
  • [x] mobile drawer menu
• translation
  • [x] content with and w/o client-side loading
  • [x] language detection (browser, cookie)
• error conditions
  • [x] 404 client-side
  • [x] 404 server-side
• basic
  • [x] page load
  • [x] page navigation
  • [x] section load
  • [x] section navigation
  • [x] check basic page elements

[tumidi]

marked #92 as a duplicate of this issue

[tumidi]

marked the checklist item TOC next/prev navigation as completed

[tumidi]

marked the checklist item section navigation as completed

[tumidi]

marked the checklist item page navigation as completed

[tumidi]

marked the checklist item page load as completed