DOCS UPDATE: Improve setup docs with dedicated CONTRIBUTING.md for local development

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

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

### What needs to be updated?

## 📌 Current Documentation Issue

Currently, multiple contributors are facing issues while setting up Ell-ena locally with full functionality.

Although some setup steps are already documented, they are scattered. It would be helpful to consolidate everything into a dedicated CONTRIBUTING.md with clear, step-by-step instructions.

## ✨ Proposed Changes

-> Add a CONTRIBUTING.md file
-> Include complete local setup steps (env, dependencies, configs, etc.)
-> Ensure contributors can run the project with all functionalities without ambiguity

This should help streamline onboarding and reduce repeated setup issues.

### Relevant Documentation Link (if any)

_No response_

### Record

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