Add tutorial for React Router v7 using library mode (#12813)

[Alto0327]

This PR adds a beginner-friendly tutorial demonstrating React Router v7 in library mode to address issue #12813.

It includes:

• Project setup and file structure
• Layout component with NavLink and Outlet
• Simple pages (Home, About, UserProfile)
• Lazy-loading with React.lazy
• Centralized route configuration with createBrowserRouter
• Suspense fallback for UX

This tutorial provides a working example for library mode routing, helping developers understand proper setup and nested routing with React Router v7.

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Latest commit: df4041b

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[remix-cla-bot]

Hi @Alto0327,

Welcome, and thank you for contributing to React Router!

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- The Remix team

[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳