Redesign shadows

[slorber]

The Infima shadows are not very convenient to use currently.

The lw shadow is not very visible, and the difference between --lw and --md is huge:

In practice, Docusaurus almost only use --lw, so it seems safe to re-adjust other shadows

Good resource on shadow design:

• https://www.joshwcomeau.com/css/designing-shadows/
• https://twitter.com/JoshWComeau/status/1458813277056688139

[slorber]

We also need a way to highlight surfaces (like cards) on dark mode, as dark shadows are not very visible.

Using a scale anim does not always look good, but using a lighter background is quite common.

See also Material design dark mode guidelines: https://material.io/design/color/dark-theme.html#properties

[slorber]

Tips from Steve Schoger (Tailwind) on how to make visible shadows in dark mode:
https://twitter.com/steveschoger/status/1499780803550658567