Replace GitBook with Clojure tooling or system that requires no local tooling

[practicalli-johnny]

To make it easier for others to contribute, replace the GitBook tooling that generates the Clojure Essentials website.

Suggestions include

• Cryogen - modifications would be required make Cryogen produce a workshop style website, rather than a blog post website.
• A new Clojure project
• a build pipeline with a test and live environment