Docs: Handle border-radii and border in an easier way

[louismaximepiton]

Description

Refactoring of the examples to have a more consistent UX.
Fixes some small issues and particularly the border-radius issues.

Motivation & Context

Ease the CSS associated to .bd-code-snippet and make the docs more consistent.
It's the first part of #38933 and #39274.

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-39328--twbs-bootstrap.netlify.app/docs/5.3/docsref

Related issues

Will help on #38933 and #39274.

[mdo]

Couple of copy changes to make it clearer in the browser what we're supposed to see with the docsref page.