chore: updates readme to direct to metamask-design-system monorepo

[georgewrmarshall]

Description

This pull request updates the README file in the design tokens repository to finalize the migration steps outlined in the [MetaMask Design System Package Migration Process Guide](https://github.com/MetaMask/metamask-design-system/blob/main/docs/package-migration-process-guide.md). Specifically, it replaces the migration notice from Step A-1 with an updated message to direct users to the new repository. The notice now informs users that the repository has been archived and future development will occur in the [MetaMask Design System repository](https://github.com/MetaMask/metamask-design-system).

Related issues

Fixes: N/A

Manual testing steps

1. Navigate to the source repository.
2. View the updated README file to confirm the migration notice is properly formatted and includes the correct links.

Screenshots/Recordings

Before

After

Pre-merge author checklist

• I’ve followed [MetaMask Coding Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).

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 or screenshots.