chore: update README to prefer pnpm over yarn and npm

[A-yushupadhyay]

This PR updates the README to emphasize the use of pnpm for installation and scripts instead of yarn or only npm.
It aligns with the current setup in the repo and helps future contributors avoid confusion.

Closes issue #1597.

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[priyankarpal]

hello @A-yushupadhyay in our readme there are still yarn & npm present so we have to remove & add pnpm

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There hasn't been any activity on this pull request recently, and in order to prioritize active work, it has been marked as stale. This PR will be closed and locked in 7 days if no further activity occurs. Thank you for your contributions!