docs: add GitHub Pages demo badge

Author: fgirolami29

README.md

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 > 🚗 European license plate validator (Russia excluded). Multi-country, regex-based validation for EU/EEA license plates. Works in **Node.js**, **TypeScript**, and the **browser** (with a lightweight UI client).
 
-[![Lang: IT](https://img.shields.io/badge/docs-Italiano-green?style=for-the-badge)](./README_IT.md)
+[![Lang: IT](https://img.shields.io/badge/docs-Italiano-green?style=for-the-badge)](./README_IT.md) [![Demo](https://img.shields.io/badge/demo-GitHub%20Pages-0b6efd?style=for-the-badge)](https://codecorntech.github.io/euro-plate-validator/)
 
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README_IT.md

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 > 🚗 Validatore di targhe europee (Russia esclusa). Validazione della sintassi basata su espressioni regex multi-paese per targhe UE/SEE. Funziona in **Node.js/TypeScript** e nel **browser** (UI client leggera).
 
-[![Lang: EN](https://img.shields.io/badge/docs-English-red?style=for-the-badge)](./README.md)
+[![Lang: EN](https://img.shields.io/badge/docs-English-red?style=for-the-badge)](./README.md) [![Demo](https://img.shields.io/badge/demo-GitHub%20Pages-0b6efd?style=for-the-badge)](https://codecorntech.github.io/euro-plate-validator/)
 
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