Docs: Harmonize the flex doc

[louismaximepiton]

Description

Adding some already existing classes instead of harcoded CSS.

Motivation & Context

Have the same rendering for all the examples displaying some flex containers.

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-37899--twbs-bootstrap.netlify.app/docs/5.3/layout/columns#horizontal-alignment
• https://deploy-preview-37899--twbs-bootstrap.netlify.app/docs/5.3/utilities/sizing
• https://deploy-preview-37899--twbs-bootstrap.netlify.app/docs/5.3/utilities/spacing#horizontal-centering

Related issues

NA