Contain all page content in landmarks

[atsansone]

From https://buganizer.corp.google.com/issues/337316892

Observed Results

Navigating through the page, all page content should be contained by landmarks.

• Screencast ChromeVox
• Screenshot

Expected Results

Ensures all page content is contained by landmarks

User Impact

Screen reader users are not able to skip to the primary content efficiently and save them time and action presses.

Steps to Reproduce

1. Enabled ChromeVox using Ctrl + Alt + Z.
2. Open the following page: https://docs.flutter.dev/
3. Start the landmark navigation using "search plus ;"
4. Observe the behavior
5. Open parashu and run the automatic tool

GAR Assessment

Flutter Docs

GAR Details

2.8. Do you provide a bypass for headings and content that are repeated across multiple pages or within the same page?

Environment Details

Machine: Chromebook / Macbook Browser: Chrome (123.0.6312.112 (Official Build) (64-bit)) Accessibility Tool: Chromevox / VoiceOver Bug filling time: 15 mins.