3.1 What is React?
Add React from CDN

# Background
React is a third-party open-source JavaScript library available on NPM.
We would normally use NPM to install third-party dependencies like React, but for really simple use cases we can also use a special CDN called UNPKG.
unpkg is a fast, global content delivery network for everything on npm. Use it to quickly and easily load any file from any package using a URL
# Requirements
Add script tags to load the react
and react-dom
required modules from these URLS:
- https://unpkg.com/react@16.13.1/umd/react.development.js
- https://unpkg.com/react-dom@16.13.1/umd/react-dom.development.js