docs: enhance README with comprehensive setup and contribution guidel…

[Muawiya-contact]

Docs: Enhance README with clearer setup and contribution guide

This PR updates the Open Visualization README to make it easier for new contributors to get started. It improves clarity around project setup, scripts, environment variables, and contribution workflow.

What’s changed

• Updated Prerequisites with Node.js (Volta optional) and Yarn versions
• Improved Getting Started instructions for cloning, installing dependencies, and running the dev server
• Added clear steps for setting up environment variables (.env.example → .env.local)
• Explained all available scripts and what they do
• Linked the Environment Variables section to avoid repetition
• Updated Contributing guide with a step-by-step workflow and guidance for larger changes
• Added useful links to Next.js, Prismic, and OpenJS Foundation resources
• Minor wording tweaks for readability and consistency

Why this is useful

• Helps new contributors get started faster and with fewer mistakes
• Follows open-source best practices for Next.js projects
• Makes documentation cleaner, more professional, and easier to follow

Other notes

• Only README was changed
• .env.example referenced for local setup
• .env*.local ignored via .gitignore
• No changes to the app’s code or functionality

Checklist

• Documentation-only change
• Easy to understand
• No breaking changes
• Follows project style

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#closes 41

[vercel]

@Muawiya-contact is attempting to deploy a commit to the OpenJS Foundation Team on Vercel.

A member of the Team first needs to authorize it.

[Muawiya-contact]

@brianleroux @voxpelli @ljharb Can you tell me, please, how to contribute to open-visualization repo? I find several improvements here.

[voxpelli]

@Muawiya-contact I have no idea