Add build/setup instructions to README.md (#40431)

[Varshad-Potle]

📖 Documentation

Closes #40431

Added a new “Building AMP Locally” section to README.md to help new contributors set up their environment.
This section explains how to install dependencies and use AMP’s task runner commands:

• npm install — install project dependencies
• npx amp build — build the runtime
• npx amp serve — run a local development server

This improves onboarding clarity for first-time contributors and replaces missing build/setup context that was previously undocumented.

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🆕 First-time contributor: Please let me know if the formatting, section placement, or command descriptions need adjustment.