docs: add useLightningcss config (#69161)
## Why?
We don't have any documentation about the experimental `next.config.ts`
config `useLightningcss`.
- Fixes: https://github.com/vercel/next.js/issues/69030
Co-authored-by: Delba de Oliveira <32464864+delbaoliveira@users.noreply.github.com>
