Add new learning resource to "Learn" page

[pedropark99]

This PR adds a hyperlink to the project "Introduction to Zig: a project-based book" into the "Learn" page of the ziglang.org website.

You can know more about this project at: https://github.com/pedropark99/zig-book

If this PR have any problems in it, then let me know and I will fix it 😉 Thank you for your attention!