docs: add React Router v6 to v7 migration guide

[Ankit-raj-11]

Add a comprehensive migration guide for React Router v6 to v7 upgrade.

This guide documents the community-built codemod (react-router-v6-to-v7) that automates the migration with zero false positives using AST-based transforms via ast-grep.

The codemod handles 4 key transforms:

• Import rewriting: react-router-dom → react-router
• Future flag injection: Smart-merges all 6 mandatory v7 flags
• API modernization: Unwraps deprecated json()/defer() calls
• Package migration: Updates package.json dependencies

Validated on real-world repos (react-admin, react-petstore) with 0 false positives across all validations.

Registry: https://app.codemod.com/registry/react-router-v6-to-v7
Source: https://github.com/Ankit-raj-11/react-router-v6-to-v7
Case Study: https://dev.to/ankit_raj_16a4c518f4c1689/automating-react-router-v6-to-v7-migration-with-ast-codemods-2888

📚 Description

🔗 Linked Issue

🧪 Test Plan

📄 Documentation to Update

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