Refactor CSS

[DavidOliver]

@josevalim, I've started with admonition.css.

Nesting consolidation, less repetition of variants' property value-setting via custom properties and in-lining of @media queries, resulting in fewer lines, a saving of 1.7kB ( 😉 ) and hopefully easier reading and editing.

Are you happy for me to go ahead with the other CSS files?

[github-actions]

📦 Docs artifacts are ready: https://github.com/elixir-lang/ex_doc/actions/runs/25527428333/artifacts/6868538334

[DavidOliver]

I've realised that for the few files that are used in the EUPB styles, we should avoid/remove nesting & selectors and unnecessary use of :is(), because support for them is not good in ebook readers.

code.css now takes this approach: with the exception of rules that already use :has(), that merely refine links with code in them and which can apply just to HTML, the rules now do not use & or :is().

I'm adding a comment to this effect at the top of such files.