docs: add comprehensive CONTRIBUTING.md guide

[praniketkw]

📄 Description

I noticed the bug report template references contributing guidelines, but the repository didn't have a CONTRIBUTING.md file. I've created one to help future contributors understand the project setup, development workflow, and submission process.

The guide covers local environment setup, coding standards, testing requirements, and the PR process. I've also included information about the project's architecture and security considerations since this handles sensitive research data.

✅ Checklist

• Tests added/updated where needed
• Docs added/updated if applicable
• I have linked the issue this PR closes (if any)

🔗 Related Issues

No existing issue. This is a documentation improvement I identified while exploring the project.

💡 Type of change

Type	Checked?
🐞 Bug fix	[ ]
✨ New feature	[ ]
📝 Documentation	[x]
♻️ Refactor	[ ]
🛠️ Build/CI	[ ]
Other (explain)	[ ]

🧪 How to test

1. Review the CONTRIBUTING.md file for accuracy and completeness
2. Verify that the setup instructions match the README
3. Check that all internal links work correctly
4. Confirm the content aligns with the project's existing documentation

📝 Notes to reviewers

This is my first contribution to the project. I tried to match the formal tone appropriate for a research collaboration and included details specific to this project like the domain-driven architecture and HIPAA/HUD compliance requirements.

Feel free to suggest any changes or additions. I'm happy to revise based on your feedback.