Automate build and deploy of Workshop website

[practicalli-johnny]

UPDATE: GitBook.io open source tool has not been maintained for many years, so this content will be migrated to MkDocs.

The plan is to create a Practicalli Learn Clojure book with the content written by John Stevenson and deprecate the ClojureBridge London workshop site.

Original Plan

Add GitHub actions to automate the building, verification and deployment of the workshop content on GitHub pages

Tasks

• Add GitHub actions to build and deploy the workshop content from this repository to GitHub pages (see practicalli/clojure for an example)
• Configure GitHub pages to server from gh-pages branch
• Configure domain (if we have one) and set HTTPS connection
• Test GitHub pages are building and being served correctly
• Remove the clojurebridgelondon/workshop repository (currently holds the generated output)
• Rename clojurebridgelondon/workshop-content-gitbook to clojurebrigelondon/workshop
• Test that https://clojurebridgelondon.github.io/workshop is working correctly