They are generated during the build process. You would need to have a compiler installed (on a Mac use Xcode to do this), also ruby (I used ruby-2.4.1) and would have to run the following sections of the build process.
download the Sonic Pi source from github to a suitable location using
git clone https://github.com/samaaron/sonic-pi.git
(This installs a folder sonic-pi containing the source code)
cd sonic-pi/app/gui/qt
…/…/server/bin/compile-extensions.rb
…/…/server/bin/i18n-tool.rb -t
cp -f ruby_help.tmpl ruby_help.h
…/…/server/bin/qt-doc.rb -o ruby_help.h
(You can see a reasonably recent complete install script I have used for Mac here
You will find a folder book has been created in the qt folder. This contains the html files to run the variouls sections of documentation eg
Sonic Pi - Examples.html
Sonic Pi - Fx.html
Sonic Pi - Lang.htmll
Sonic Pi - Samples.html
Sonic Pi - Synths.html
It also uses a folder theme which is also in the qt folder.