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

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