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.

[CLAassistant]

All committers have signed the CLA.

[Varshad-Potle]

🆕 Hi, I’m a first-time contributor to AMP.
I’ve added the new “Building AMP Locally” section to the README as discussed in issue #40431.
Please let me know if the formatting, section placement, or descriptions need any adjustments.

[Kaushal-15]

Hi! I was going through this PR as a new contributor.
The updated build/setup instructions look great — the steps using npm install, npx amp build, and npx amp serve are clear and easy to follow now.
This definitely improves the onboarding experience for first-time contributors.

Thanks for the detailed update! 🙌

[Varshad-Potle]

Thank you! Glad the updated steps made things clearer.
If you notice anything else that could be improved, feel free to ping me.