[Docs] Avatar family migration documentation (7 components)

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## Summary

Add migration documentation for the Avatar component family (7 components) that have been migrated from Extension/Mobile to MMDS.

## Components

- AvatarBase
- AvatarIcon
- AvatarAccount
- AvatarNetwork
- AvatarToken
- AvatarFavicon
- AvatarGroup

## Definition of Done

### 1. MIGRATION.md Sections
Add migration sections with API maps to:
- `packages/design-system-react/MIGRATION.md`
- `packages/design-system-react-native/MIGRATION.md`

Include:
- Breaking changes between Extension/Mobile and MMDS
- Prop mapping tables
- Before/after code examples

### 2. README Links
Add migration section to each component's README.mdx:
- Link to relevant MIGRATION.md section

### 3. @deprecated Updates
Add migration links to JSDoc in:
- Extension: `ui/components/component-library/avatar*`
- Mobile: `app/component-library/components/Avatar*`

## Example Reference
Text component migration: https://github.com/MetaMask/metamask-design-system/blob/main/packages/design-system-react/MIGRATION.md#text-component

## Labels
documentation

Start with the Text component migration section in packages/design-system-react/MIGRATION.md, then inspect the React and React Native Avatar README.mdx files and the listed Extension and Mobile component paths. Document all seven Avatar components in both MIGRATION.md files, link each README and deprecated JSDoc entry, and verify that the API maps, breaking changes, and before/after examples are complete.

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