feat: add comprehensive Developer Guide and enhance API documentation

[mohyware]

• Introduced a new DEVELOPER_GUIDE.md file detailing project setup, contributing guidelines, and development processes.
• Created openapi.json for API documentation, detailing endpoints and features.
• Refactored payload models with clearer structure and field descriptions for better understanding, and improved route documentation for spectrum calculation and fitting endpoints with detailed explanations and examples.

[vercel]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Hits            ?      378           
  Misses          ?      148           
  Partials        ?       12           

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