📖 Add Hackathon Video and Testnet Link to README

📌 Description

This PR adds a hackathon demonstration video and testnet deployment link to the README.md file to showcase the StarkLotto project and provide users with a live testnet version for testing.

🎯 Motivation and Context

The changes are necessary to:

• Showcase the project with a demonstration video from the hackathon
• Provide users with easy access to the testnet version for testing
• Improve project visibility and user engagement
• Document the live deployment for community access

🛠️ How to Test the Change (if applicable)

1. 🔹 Navigate to the repository README.md file
2. 🔹 Verify the YouTube video embed displays correctly
3. 🔹 Click the testnet link to ensure it redirects to the live application
4. 🔹 Confirm the video thumbnail and link formatting are properly rendered

🖼️ Screenshots (if applicable)

The changes include:

• YouTube video embed with thumbnail: https://www.youtube.com/watch?v=Lra23dFcTMw
• Testnet deployment link: https://starklotto-eta.vercel.app/

🔍 Type of Change

• 📖 Documentation - Updates or creates new documentation.

✅ Checklist Before Merging

• 🧪 I have tested the code and it works as expected.
• 🎨 My changes follow the project's coding style.
• 📖 I have updated the documentation if necessary.
• ⚠️ No new warnings or errors were introduced.
• 🔍 I have reviewed and approved my own code before submitting.

📌 Additional Notes

This documentation update enhances the project's presentation by adding visual demonstration content and providing immediate access to the testnet version. The changes are minimal and focused on improving user experience and project visibility without affecting any code functionality.