Docs: fix margins between cards in examples

[julien-deramond]

Description

This PR adds margins in some cards examples in the documentation to avoid having cards "glued" together.

Motivation & Context

Better rendering at all breakpoints.

Type of changes

• Enhancement (non-breaking change which fixes an issue)

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-37493--twbs-bootstrap.netlify.app/docs/5.2/components/card/#using-grid-markup (small breakpoint)
• https://deploy-preview-37493--twbs-bootstrap.netlify.app/docs/5.2/components/card/#using-utilities
• https://deploy-preview-37493--twbs-bootstrap.netlify.app/docs/5.2/components/card/#text-alignment