docs: rewrite README with Homebrew install, accurate version, and expanded getting-started guide

[devin-ai-integration]

Summary

Rewrites the README to fix several inconsistencies and gaps reported by a customer:

Fixed inconsistencies:

• Version number in example was 4.21.3 but actual fern.config.json is 5.35.4
• JSON example used invalid comment syntax (// fern.config.json)

Added content:

• Homebrew install option (brew install fern-api) alongside npm — this was a specific customer ask
• Repository structure diagram showing all key files and their purpose
• fern check validation step before previewing
• CI/CD workflows section documenting the three included GitHub Actions (check, preview, publish) and how to set up FERN_TOKEN
• fern init --docs as an alternative from-scratch path
• Useful commands quick-reference table
• Further reading links to official docs
• Preview URL generation (fern generate --docs --preview)

The getting-started flow is now a clear 5-step numbered walkthrough: create repo → configure org → validate → preview → publish.

Link to Devin session: https://app.devin.ai/sessions/ec2afc13d91d4982b33b674c1628437f

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://plantstore-preview-devin-1781039284-update-readme.docs.buildwithfern.com