CONV-327: Add developer quickstart section

[menvil]

Summary

• Adds 7-step developer quickstart to /docs/api page
• Documents the full happy-path flow: authenticate → upload → estimate → convert → poll → download
• Includes auth note about plan requirement

Test plan

• Test written first — fails before quickstart content added
• composer test passes (616 tests)

🤖 Generated with Claude Code

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Summary by cubic

Implements CONV-327 by adding a 7-step developer quickstart to /docs/api that guides users from authentication to downloading a converted file.

• New Features
  • Documents the happy-path flow and key endpoints, including polling.
  • Adds an auth note with the required header and Pro/Max plan requirement.

^{Written for commit 84c88a1. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/CONV-327-add-developer-quickstart-section

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