Improve README for easier setup and contribution

[Muawiya-contact]

Problem

New contributors may find it confusing to set up the project or understand how to contribute.

• The .env.local setup is not clear.
• Available scripts and workflow are not fully explained.
• Contribution steps could be more beginner-friendly.

Solution

• Update README with clear, step-by-step instructions:
  • Clone, install, and run the project
  • Set up environment variables using .env.example → .env.local
  • Explain all scripts and their purpose
  • Provide a clear contribution workflow
• Add links to Next.js, Prismic, and OpenJS resources
• Improve readability and consistency

This will help new contributors start faster, reduce errors, and follow standard OSS practices.