[Docs] Add README migration links for existing MIGRATION.md sections

## Summary

Several components already have MIGRATION.md sections but are missing links from their README files. This ticket is to verify and add those missing links.

## Components to Check

### React (README.mdx)
- Box - Check if migration link exists, add if missing
- BannerAlert - Check if migration link exists, add if missing
- Text - ✅ Already has link
- Icon - ✅ Already has link

### React Native (README.md)
- Same as above

## Verification Steps

1. Check each component's README for a "Migration from [Extension|Mobile] Component Library" section
2. If missing, add section linking to MIGRATION.md#component-name
3. Use Text component README as reference: https://github.com/MetaMask/metamask-design-system/blob/main/packages/design-system-react/src/components/Text/README.mdx#L574-L576

## Reference Pattern

```markdown
## Migration from Extension Component Library

For detailed migration instructions from the Extension component-library, see the [Migration Guide](https://github.com/MetaMask/metamask-design-system/blob/main/packages/design-system-react/MIGRATION.md#component-name).
```

## Labels
documentation

Start by checking the React README.mdx and React Native README.md files for Box and BannerAlert, using the existing Text component README section as the reference. Compare each component with its corresponding section in MIGRATION.md, add only missing migration links, and verify that every listed component points to the appropriate MIGRATION.md#component-name anchor.

Written by the indexing model from the issue text.