docs: add prerequisites to Development section

[alexgrozav]

Resolves STY-12

Summary

• Add Node.js (≥22) and pnpm (≥10.7.1) version requirements to the Development section
• Add cd styleframe and pnpm dev:docs commands to help new contributors get started
• Link to CONTRIBUTING.md for detailed contribution guidelines

Test plan

• Verify README renders correctly on GitHub

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5d0d274

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Project	Deployment	Actions	Updated (UTC)
storybook-styleframe-dev	Ready	Preview, Comment	Jun 24, 2026 4:00am
styleframe-dev	Ready	Preview, Comment	Jun 24, 2026 4:00am

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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