docs(css): add Tailwind v4 Vite plugin setup guide

[Bohdan08]

Summary

The CSS docs currently only show the Tailwind v3 PostCSS approach. Tailwind v4 ships a dedicated Vite plugin (@tailwindcss/vite) that replaces PostCSS entirely so no postcss.config.js or tailwind.config.js needed.

Since Ladle defaults to Vite's CSS handling, passing the plugin in vite.ladle.config.ts is all that's required.

Changes

• Adds a Tailwind v4 subsection above the existing v3 content so both versions are documented
• Renames the existing content to Tailwind v3 for clarity

Why

New projects default to Tailwind v4. Without this, users hit a gap in the docs and have to figure out the Vite plugin path themselves. the PostCSS-based setup from the v3 guide doesn't apply and can cause confusion.

Checklist

• Docs-only change, no code modified
• Both v3 and v4 setups remain documented