Docs: add <main> landmark to homepage masthead

[TanmayNileshShinde]

Description

Updates the homepage masthead wrapper in site/src/components/home/MastHead.astro from a generic

to a landmark with id="content", and passes slot="main" in site/src/pages/index.astro.

Motivation & Context

Fixes #43045.

As noted in #43045 and flagged by accessibility checks (Lighthouse / axe-core landmark-one-main), the v6 homepage lacked a primary

landmark.

Using

provides the required landmark for assistive technologies and correctly grounds the skip link (href="#content") without altering visual styles.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Related issues

Closes #43045

[julien-deramond]

Thanks for your PR @TanmayNileshShinde but I used a slightly different approach via #43105