Add custom variant as an alternative to docs

[kidonng]

As per discussion in #568.

The section hints at changing specificity since there can be many ways to approach it (like using @layer), but only :where() is listed as an example as that's what I find most straightforward.

[dcastil]

Thanks for the addition! Just need to fix the comment syntax, but otherwise good to go.

[codspeed-hq]

CodSpeed Performance Report

Merging #592 will not alter performance

_{Comparing kidonng:patch-1 (f06f9df) with main (40d8fee)}

Summary

✅ 5 untouched benchmarks

[github-actions]

This was addressed in release v3.4.0.