Add Descriptions to links in the Introduction

[tuantinghuang]

You can find the full version of the code on the book’s website. There, you can interact with, modify, and experiment with the code in the p5.js Web Editor. Additionally, everything is included in the book’s GitHub repository. Here are the full URLs to access all the materials:

• https://natureofcode.com
• https://github.com/nature-of-code
• https://editor.p5js.org/natureofcode/sketches

Once all the website info is solidified, this section may benefit from a brief explanation of what exactly is on each site (and maybe how to find it, unless it's so self-explanatory once the reader gets to the site)

[shiffman]

This has been completed as part of my work on #582