Docs: add Border radius section for Input group

[TommasoAllegretti]

Description

I added a Border radius section in the Input group docs to explicitly show how that style is applied to the children elements.

I might have given too much importance to the section because I wanted the behavior to be clear to newcomers and avoid any confusion, but I could see how the same information could be delivered just by adding a single line in the Basic example section.

Please let me know if there is anything I can improve with my solution.

Type of changes

• Documentation enhancement (non-breaking change)

Checklist

• [x] I have read the contributing guidelines
• [x] My code follows the code style of the project (using npm run lint)
• [x] My change introduces changes to the documentation
• [x] I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• [x] All new and existing tests passed

Live previews

• https://deploy-preview-40776--twbs-bootstrap.netlify.app/docs/5.3/forms/input-group/#border-radius

Related issues

Closes #40763