Use Starlight `<LinkButton>` instead of our `<Button>`

[julien-deramond]

Note

No longer relevant: the site left Starlight in #1686. <Button> now comes from @deramond.dev/astro. (2026-09-28)

Description

We've created a custom <Button> component while refactoring the website with Starlight. Along the way, Starlight released a new <LinkButton> component as well.
Let's study whether we can replace our <Button> by this <LinkButton>.