Collapsible Proofs out of the box

[FelixBenning]

Description

I would like Quarto to provide collapsible proofs out of the box. The current suggestion is to use callouts (#5272 (comment))
and there is the Extension Callouty Theorems that wraps the Proofs into a callout, but this feels more like a hack:

The callout box makes the proof stand out. This is what you want for callouts, but this is not really what you want for proofs. Proofs are the details whearas theorems should be highlighted. This forces you to use Callouts for Theorems too to "balance" them visually. But then

1. the Callout is either still called "Important" or similar, or you allow Callouty Theorem to copy the title of the Theorem and you have duplicate titles.
2. When the Theorem is referenced it will only show the content and not the "styling" on hover
3. Everything is a callout, which is too noisy visually

It would be really nice if proofs could be a bit more subtle/tasteful collapsibles.