chore: updating readme with migration notice

[georgewrmarshall]

Description

This PR adds a migration notice to the README of the MetaMask Design Tokens repository, indicating that the repository is being migrated to the MetaMask Design System repository.

Related issues

Fixes: #743
Related: MetaMask/metamask-design-system#88

Manual testing steps

1. Navigate to the repository’s README.
2. Verify that the migration notice is present and links correctly to the new repository and migration guide.

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed MetaMask Coding Standards.
• I've clearly explained what problem this PR is solving and how it is solved.
• I've linked related issues.
• I've included manual testing steps.
• I've included screenshots/recordings if applicable.
• 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).

Pre-merge reviewer checklist

• I've manually tested the PR.
• I confirm that this PR addresses all acceptance criteria and includes necessary testing evidence, such as recordings or screenshots.