Use blockquote styles for collapsible-blockquote

[expikr]

[ehuss]

Can you say why this would be added instead of putting the <details> block inside a blockquote? For example:

CLICK ME

Content goes here.

In general, when opening a pull request, it would be helpful to include some text explaining what a PR does and what the motivation is behind it.

[ehuss]

@rustbot author