Docs: use concrete checks and radios aria labels

[Kevinjohn]

Description

Replaces placeholder accessible names in the checks and radios "Without labels" example.

Motivation & Context

The example explains that controls without visible labels still need an accessible name, but the copied markup used literal aria-label="..." values.

Expected: the example uses concrete accessible names.
Actual: the example exposes ... as the checkbox and radio accessible names.

This updates the example to use concrete labels for the checkbox and radio inputs.

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 (using npm run lint)
• 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

Live previews

• https://deploy-preview-42708--twbs-bootstrap.netlify.app/

Related issues

None.

Checks

• git diff --check
• npm exec -- prettier --config site/.prettierrc.json -c site/src/content/docs/forms/checks-radios.mdx
• npm run docs-build