docs: add .env.example and document environment configuration

[manshusainishab]

Closes #749

Summary

Adds .env.example and updates README to document environment configuration.

Changes

• Added .env.example with placeholders for all supported environment variables
• Updated README with setup instructions
• Categorized variables for clarity

Why

Improves onboarding and follows standard open-source configuration practices.

Testing

No runtime logic changed. Documentation-only improvement.

[manshusainishab]

@northdpole if this feels a improved onboarding experience please review. I am throughly reviewing the onboarding process as you mentioned in
Repository Health Analysis Report: OWASP/OpenCRE
First Time contributor no. is 0 which is severely high and I feel one of the reasons is the onboarding experience of NEW CONTRIBUTORS as they are just starting their open source journey.

[manshusainishab]

@northdpole is this feels a improvement or should we just close the PR.

[northdpole]

this is a good idea, let's also add the ability to load the dotenv when we run the service at least locally

[manshusainishab]

hey @northdpole,
done with changes and also will let you know about the module B pre-experiment

[manshusainishab]

@northdpole updated the branch ready for the review

[manshusainishab]

hey, @northdpole its a quick reminder to please review or if any further changes required I am happy to make them.