docs: Improve README and onboarding instructions for local development setup

[PunyaGowdagd66]

** Overview**

This PR improves the README and onboarding documentation to make the local development setup clearer and more beginner-friendly for new contributors.

** Changes Made**

• Updated Node.js version recommendations (v18+ recommended, v20+ preferred)
• Added a verification step for Node.js, npm, and Git installation
• Clarified the difference between development and production setup
• Corrected the production build output directory to .next
• Improved formatting and wording for better readability

** Purpose**

Several new contributors face difficulties while setting up the project locally.
These updates aim to provide clearer instructions and reduce onboarding friction.

** Type of Change**

• Documentation update

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[PunyaGowdagd66]

Hi @M4dhav,

I noticed this issue was previously assigned, but since there was no recent update, I worked on improving the README and onboarding documentation to make the local setup clearer for new contributors.

If this overlaps with ongoing work, I’m happy to make adjustments or close the PR.

Thank you.