☔ Introduce a simplified, accessible site design with support for dark mode

[parlough]

Design and accessibility related work on the website frontend to finish in Q1. Many of these tasks will be completed on dart.dev as well.

Clean up and modernization:

• ☔ Migrate to Bootstrap 5 or remove it #8030
• Remove jquery once our own usages and bootstrap are removed #11249
• Implement more efficient layout system that doesn't rely on JS

Accessibility:

• ☔ Update website for accessibility #9495
  • Contain all page content in landmarks #10543
  • Fix clipped text in footer in responsive design #10532
• "Add Cursor Pointer for Search Icon on Website" #9329
• Add a consistent focus-visible style to all applicable elements

Enhancements:

• Add a consistent experience for navigating between Flutter-related sites #11658
• Implement new footer that aligns with dart.dev to surface social links and policies
• Add dark mode to the official documentation #3632
• Add banner that can be hidden on scroll #11710
• TOC accessible from narrow/mobile layout

Miscellaneous usability improvements:

• Code snippets are impossible to read due to 10 letter visible column on mobile #2313
• Page TOC should support auto-scrolling #1795
• Page overscroll doesn't scroll the whole page #8505
• Jumping viewport when using leftnav #1728
• Consistent and accessible cards across the website