Theme: Add Tokens "Introduction" Storybook page

[aduth]

What?

Updates Storybook to add a "Design System/Tokens/Introduction" page that surfaces content from @wordpress/theme's tokens.md.

Related: #78438

Why?

Paired with #78438, this document is becoming a canonical reference for what tokens exist and how to use them. Currently it lives as a document in the package but isn't surfaced anywhere outside the code or GitHub repository view. Surfacing it in Storybook makes it more accessible for human readers.

How?

Adds a new .mdx Story file that mirrors to the existing file.

Advantage over renaming the file from .md to .mdx:

• Doesn't expose MDX metadata when viewed through GitHub
• Maintains support for Design System MCP, which references the tokens.md file directly from its GitHub URL

Testing Instructions

Verify in Storybook:

1. npm run storybook:dev
2. Go to http://localhost:50240/?path=/docs/design-system-tokens-introduction--docs
3. Observe same content as in tokens.md

Screenshots or screencast

Use of AI Tools

Claude Code with Opus 4.7. Manual intervention to shift code to packages/theme out of root storybook/ and exempt ESLint rules.

[github-actions]

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Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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[github-actions]

Flaky tests detected in 023fea1.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/26163358045
📝 Reported issues:

• [Flaky Test] Navigates the items list via UP/DOWN arrow keys #64334 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js

[ciampo]

Nice!