Docs: fix highlight border radius

Description

This PR tries to fix the highlight border radius in our docs coming from https://github.com/twbs/bootstrap/commit/20ab8219a0caca784483e93083cde269055d0f3a.

⚠️ But maybe there is a smarter/better way to do it. IMO the rendering in this PR is better but the values are not exactly the same depending on the cases; sometimes 7px, sometimes 8px.

Issues

(Shoutout to @Hideoo for catching it)

Upper border radius of the "simple" highlight is straight in our main branch contrary to the live site.

But in some cases with more complex rendering, it was OK like in https://twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#example

OK but still with a small issue (in https://twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#flush for example):

Type of changes

• Bug fix (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
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38114--twbs-bootstrap.netlify.app/

Plus examples or more complex use cases in:

• https://deploy-preview-38114--twbs-bootstrap.netlify.app/docs/5.3/getting-started/introduction/
• https://deploy-preview-38114--twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#example
• https://deploy-preview-38114--twbs-bootstrap.netlify.app/docs/5.3/components/accordion/#flush