Fixed many accessibility issues. (#20990)
### Description
Resolving or re-resolving: #20096 , #20118 , #20153 , #20151 , #20152
Done by:
- Adding skip to main content links
- Adding pause carousel
- Fixing H1s
- Fixing color scheme
- Making code blocks appear on mobile / smaller viewports.
### For testing
Please test using mobile and desktop versions, ensuring everything
(especially the landing page) appear as expected!
