Docs: Add main element to Cheatsheet Examples

[coliff]

Description

Fixes an accessibility issue reported by Axe Core.
REF: https://dequeuniversity.com/rules/axe/4.8/landmark-one-main

Motivation & Context

Makes is easier for accessibility tools to navigate by landmarks

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-40577--twbs-bootstrap.netlify.app/

Related issues

[julien-deramond]

LGTM, thanks @coliff for the patch.
I'm handing the PR over to @patrickhlauke for a11y final check, just to be 100% sure 😇

[patrickhlauke]

Noting that it's only a best practice, not an actual failure...but sure