Create migration guide and update documentation for React 18

## Description

Create comprehensive documentation for the React 18 migration, including migration guides, breaking changes, and upgrade instructions for users currently on React 17.

## Content to Create/Update

### Migration Guide
- Step-by-step upgrade instructions for React 17 → React 18
- Breaking changes documentation
- Before/after code examples
- Common issues and solutions

### Documentation Updates
- Update README files to reflect React 18 requirement
- Update API documentation
- Update cookbook documentation
- Update contributing guidelines

### Breaking Changes to Document
- React 17 no longer supported
- Legacy interopt package removed
- Deprecated APIs removed (renderComponent, renderApp)
- New React 18 APIs required

## Implementation Steps
1. Create migration guide document
2. Update package READMEs
3. Update main repository README
4. Update API documentation
5. Add breaking changes section to CHANGELOG
6. Update cookbook documentation

## Files to Update
- `README.md`
- `packages/react/app/README.md`
- `packages/react/README.md`
- `CHANGELOG.md` (breaking changes section)
- `contributing/` documentation
- Create new migration guide (e.g., `MIGRATION_GUIDE.md`)

## Labels
📚 documentation, 👾 React, 🧨 breaking changes

Review the existing README.md, packages/react/app/README.md, packages/react/README.md, CHANGELOG.md, contributing/ documentation, and cookbook and API documentation before creating MIGRATION_GUIDE.md. Document the React 17-to-18 upgrade, breaking changes, removed APIs, required APIs, examples, and common issues, with all listed documentation and README updates completed.

Written by the indexing model from the issue text.