Add markdown documentation builder for design tokens

[Copilot]

Description

Adds a complementary builder that generates lib/tokens.md documenting all design tokens with their resolved values. Helps developers quickly look up actual values when seeing token references like context.color.fg.primary.base in code.

Changes:

• New MarkdownDocumentationFactory builder outputs tokens.md with:
  • Tokens organized by set (global, light, dark, etc.)
  • Full token paths, types, and resolved values
  • Aliases and math expressions pre-evaluated via existing prepareTokens()
• Registered as markdown_documentation_builder in build.yaml
• Tests using build_test framework

Usage:

targets:
  $default:
    builders:
      design_tokens_builder:design_tokens_builder:
        enabled: true
      design_tokens_builder:markdown_documentation_builder:
        enabled: true

After running build_runner, lib/tokens.md contains entries like:

- **global.fontSize.sm**
  - Type: `fontSizes`
  - Value: `13.0`

Related issues & PRs

No related issues or PRs found.

Checklist

• I added a PR description.
• I linked all related issues and PRs I could find (no links if there are none).
• If this PR changes anything about the main design_tokens_builder package (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change on its own is not worth an update).
• If this PR includes a notable change in the design_token_builder package, I updated the version according to Dart's semantic versioning.
• If there is new functionality in code, I added tests covering all my additions.
• All required checks pass.

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Original prompt

Help me build a complimentary output builder that generates a Markdown file that lists all tokens that were generated in a list with the resolved value.

This should help document what the values are when you are seeing something like:
context.color.fg.primary.base
in the codebase.

---

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

✅ Prusdrum
✅ IchordeDionysos
❌ Copilot
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Sese-Schneider]

RSLGTM