docs: Add .env template and environment variables setup guide

[NobleCoder69]

Problem Statement

Currently, developers don't know which environment variables are required to run GA4GH-RegBot. Without clear guidance, they:

• Don't know what API keys are needed
• Don't know where to store sensitive information
• Risk committing secrets to Git
• Waste time troubleshooting missing env vars

Objective

Create a .env.example template and comprehensive guide so developers can quickly set up their environment.

Acceptance Criteria

1. Create .env.example with:

   • All required environment variables
   • Placeholder/example values
   • Comments explaining each variable
   • Security warnings where needed

2. Create ENV_SETUP.md with:

   • Step-by-step setup instructions
   • Where to get API keys (OpenAI, etc.)
   • How to load env vars in Python code
   • Common mistakes to avoid
   • Security best practices

3. Update README with:

   • Quick "Environment Setup" section
   • Link to ENV_SETUP.md

4. Update CONTRIBUTING.md with:

   • Env setup requirement
   • Reference ENV_SETUP.md

Success Metrics

✅ .env.example has all required variables documented
✅ ENV_SETUP.md is clear and comprehensive
✅ New developers can set up environment in < 5 minutes
✅ No accidental secret commits possible

Notes

• Security first: Never commit .env file
• .env.example should have placeholder values only
• Document sensitive variables clearly