Improving the appearance of the .keyfeatures boxes

[alphapapa]

As has been noted, the appearance of the .keyfeatures boxes could be improved. Especially when displayed in the dark mode theme, it leaves something to be desired.

I'd suggest redesigning them to not use shadows, which should significantly improve the appearance on dark-mode, and probably look nicer on light-mode as well.

For inspiration, we could consider the pydata-sphinx-theme, which has some boxes at the top of that page, although they seem a bit plain. I'd like to try imitating its "See also" box below, which seems like a nice balance of aesthetics and usability.

[jarrodmillman]

The boxes on this site are also fairly nice: https://learn.scientific-python.org/development/

[alphapapa]

This should be resolved for #233 according to #233 (comment). After that PR is merged, we can consider further improvements as we work on #225.