☔ Update website for GAR compliance

[atsansone]

This project covers updating the docs.flutter.dev site to meet the Google Accessibility Requirements.

This project's details can be found in go/dash-site-gar-report.

This includes the following tasks:

Assess a11y needs

• [x] #9535
• [x] #9534
• [x] #9533

Fix site wide common issues

• [x] #9532
• [x] #9427
• [x] #9523
• [x] #9707
• [x] #9708

Fix heading issues

• [x] #9524
• [x] #9525
• [x] #9526
• [x] #9527

Fix ARIA IDs and attributes

• [x] #9528
• [x] #9529
• [x] #9530

Fix lexer contrast

• [x] #9531

Core a11y Testing

• [x] #10530
• [x] #10531
• [ ] #10532
• [x] #10533
• [ ] #10534
• [ ] #10535
• [ ] #10536
• [ ] #10537
• [ ] #10538
• [ ] #10539
• [ ] #10540
• [ ] #10541
• [ ] #10542
• [ ] #10543
• [ ] #10544
• [ ] #10545