Docs: Update setup instructions to use pnpm

[mishra-krishna]

This pull request updates the README.md to recommend pnpm for dependency installation and for running the development server.

Reasoning:

The project contains a pnpm-lock.yaml file, which indicates that pnpm is the intended package manager. The previous instructions recommended npm, which could lead to several issues for new contributors:

• Dependency Mismatches: npm and pnpm can resolve dependencies differently, potentially leading to a node_modules state that is inconsistent with the lock file. This can cause subtle bugs or break the application, like the lightningcss error that was encountered.
• Incorrect Lock File: If a user runs npm install, a package-lock.json file might be created, leading to confusion about which lock file is the source of truth.
• Installation Errors: As seen, using the wrong package manager can lead to native addon installation failures if the resolution differs from what the lock file expects.

By aligning the documentation with the project's setup, we can provide a smoother onboarding experience for new developers and prevent common setup-related issues.

This change also includes a note on how to install pnpm for users who might not have it installed.