chore: updating design-tokens readme to align with other packages

[georgewrmarshall]

Description

This PR updates the README for the Design Token Package to align with its new status as a non-root package in the design system monorepo. This change follows the package migration process guide and standardizes the README structure, making it consistent with other non-root packages in the monorepo.

Related issues

Fixes: #123

Manual testing steps

1. Navigate to the README of the Design Token Package.
2. Confirm that the Introduction, Installation, Usage, and Contributing sections are present and follow the monorepo's standard structure.
3. Verify that instructions on development tasks (e.g., Test, Build) are removed.

Screenshots/Recordings

Before

before720.mov

After

after720.mov

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 changes).
• I confirm that this PR addresses all acceptance criteria and includes necessary testing evidence, such as recordings or screenshots.

[georgewrmarshall]

@metamaskbot publish-preview