docs: 📚 describe link usage for sd-button

[mariohamann]

Description

@matuzo pointed out, that the usage of a button as a link (using href, download etc.) is completely missing in the docs. The danger is high that people use the button and do evil JavaScript things to imitate behaviour of a link. This would harm a11y and is super bad practice. We therefore have to add information in the docs how to use the button as a link.

Here's a great example for that: https://shoelace.style/components/button#link-buttons

Where to put the documentation

Button

DoR

• [ ] Item has business value
• [ ] Item has been estimated by the team
• [ ] Item is clear and well-defined
• [ ] Item dependencies have been identified

DoD

• [ ] Documentation has been created/updated (if applicable)
• [ ] Migration Guide has been created/updated (if applicable)
• [ ] Relevant stories (Features, A11y) are created/updated
• [ ] Implementation works successfully on feature branch