Book pages are duplicated

[imaginator]

Duplicate documentation:

• https://github.com/BlockstreamResearch/SimplicityHL/tree/master/book/src
• https://github.com/BlockstreamResearch/simplicity-lang-org/tree/main/docs/simplicityhl-reference

My recomendation would be to consolidate the book pages on the docs website and remove them from the HL website.

[apoelstra]

Would have been closed by BlockstreamResearch/SimplicityHL#141 -- but that PR was closed without comment. Did you mean to close it?