remove book (moved to docs website) #141
                
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I've moved the book contexts to the documentation website hence they can be removed from here.
This PR also removed the build script for the book to github pages.
Please also disable Github Pages for this repo (https://github.com/BlockstreamResearch/SimplicityHL/settings/pages) so that I can publish the docs.simplicity-lang.org pages (only one Github pages per organisation limitation).