DOCS UPDATE: Add Core Project Documentation (Contributing, Code of Conduct, Changelog, API, Dev Workflow, Deployment)

### Is this related to an existing part of the documentation?

- [x] Yes, it is related to an existing section

### What needs to be updated?

## ✨ Proposed Changes

Fill critical documentation gaps to improve onboarding and collaboration:

- Add CONTRIBUTING.md and CODE_OF_CONDUCT.md
- Create CHANGELOG.md (semantic, unreleased section)
- Add API docs (OpenAPI link and usage examples)
- Write development workflow guide (setup, scripts, tests)
- Complete deployment instructions (Docker, env, services)

## 📋 Additional Context

### Relevant Documentation Link (if any)

_No response_

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this update