Add CONTRIBUTING.md and .env.example for better developer onboarding

[rachana192837]

Problem

New contributors (including GSoC applicants) have no guide on:

• How to set up the project locally beyond what's in README
• What environment variables are needed (.env.example is missing)
• Branch naming conventions, PR process, commit message style

Proposed Solution

• Add a CONTRIBUTING.md with setup steps, branch naming, and PR guidelines
• Add a .env.example listing all required environment variables with placeholder values

Why this matters

This reduces friction for new contributors and makes onboarding faster.

I'd like to work on this if maintainers agree!