Show rendered HMTL boxes in documentation

[TimTeaFan]

At the momentan the documentation (README) shows how to implement HTML boxes, but they are not visually displayed. It would be great to actually use the boxes in the documentation, but the default output format of the readme is github_document() and I'm not sure if we can switch it to hugodownplus::md_document(). If that is not possible, we could fake it with direct HTML tags <details> and HTML <summary> or just use a screenshot instead (the latter would be less cool).

[TimTeaFan]

It seems actually using hugodownplus::md_document() as output format of the README Rmd is not rendering correctly, see here.

However, we can just fake the boxes (as shown here) but I'm not sure if we can apply preset CSS styles, probably not in the rendered Github README, but we could add CSS to the extra.css file of the {pkgodown} website, then at least there we would have some sort of semi-live demo.