Docs/update readme pnpm

[hritikmittal09]

First thing, PLEASE READ THIS: ReactPlay Code Review Checklist

Description

This PR updates the README to use pnpm exclusively instead of yarn and npm for package management.

Fixes #1597

Motivation and Context

The current README still mentions npm and yarn. Since the project has standardized on pnpm, continuing to reference the other package managers may confuse new contributors.
This change ensures clear, consistent setup instructions for all developers.

Changes Made

• Replaced all yarn and npm commands with pnpm.
• Updated installation instructions for Windows, Linux, and MacOS to use pnpm.
• Updated format, lint, build, start, and test commands to use pnpm.
• Removed references to yarn from the dependencies section.
• Added screenshot showing the updated README.

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Type of change

• This change requires a documentation update

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How Has This Been Tested?

• Verified that all pnpm commands in the README work correctly on local environment.
• Ran pnpm install, pnpm start, pnpm run format, pnpm run lint, and pnpm build to ensure everything works as expected.

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Checklist:

• I have performed a self-review of my own changes.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

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Screenshots or example output

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