fix: update docs PostCSS config for Tailwind CSS v4

- Install @tailwindcss/postcss package (required for Tailwind v4)
- Create postcss.config.mjs using @tailwindcss/postcss plugin
- Fixes build error: 'tailwindcss directly as PostCSS plugin'

Author: prosdev

docs/package.json

@@ -15,6 +15,7 @@
     "react-dom": "^19.0.0"
   },
   "devDependencies": {
+    "@tailwindcss/postcss": "^4.1.18",
     "@types/node": "^22.10.0",
     "@types/react": "^19.0.0",
     "autoprefixer": "^10.4.23",