Docs: Use <article> element for example cards

[coliff]

Description & Motivation & Context

This makes semantic sense.

REF: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/article

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-40965--twbs-bootstrap.netlify.app/examples/

Related issues

[louismaximepiton]

Thanks for the contribution!
On my side, I'm fine with it, any thoughts @patrickhlauke ?

[patrickhlauke]

In practical terms, this doesn't really matter all too much (from a quick check, NVDA doesn't announce articles, though JAWS does), but sure...