chore: [DSYS-616] create/update Box migration docs

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Description

Updates Box migration documentation for both web and React Native based on direct source comparisons.

Changes included:

• Updated packages/design-system-react/MIGRATION.md Box section with a structured legacy extension (ui/components/component-library/box) to MMDS React API mapping:
  • removed/renamed props,
  • type/value changes,
  • behavior differences,
  • before/after migration examples.
• Updated packages/design-system-react-native/MIGRATION.md Box section with a structured legacy mobile to MMDS RN mapping based on audited mobile legacy Box sources (app/components/UI/Box), including explicit source-path notes where app/component-library/components/Box is absent.
• Added README migration links for Box in both packages:
  • packages/design-system-react/src/components/Box/README.mdx
  • packages/design-system-react-native/src/components/Box/README.md

DSYS_EPIC: DSYS-616
MIGRATION_DOCS_VERSION: 5d35eda

Related issues

Fixes: DSYS-629

Manual testing steps

1. Open packages/design-system-react/MIGRATION.md and verify the ### Box Component section contains structured API mapping tables and extension before/after examples.
2. Open packages/design-system-react-native/MIGRATION.md and verify the ### Box Component section contains structured API mapping tables, mobile source path notes, and RN before/after examples.
3. Open packages/design-system-react/src/components/Box/README.mdx and verify the ## Migration Guide section links to ../../MIGRATION.md#box-component.
4. Open packages/design-system-react-native/src/components/Box/README.md and verify the ## Migration Guide section links to ../../MIGRATION.md#box-component.
5. Run validation commands from repo root:
   • yarn lint:fix
   • yarn lint
     and confirm they currently fail due to pre-existing repo-wide tailwindcss/no-custom-classname errors unrelated to this PR.

Screenshots/Recordings

Before

N/A

After

N/A

Pre-merge author checklist

• I've followed MetaMask Contributor Docs
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

  

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