Please visit our guide to contribute to this project
https://github.com/rastersoft/autovala/blob/master/cmake/Valadoc.cmake
From embik or anyone wondering how to get the valadoc stuff offline: Install devhelp and download the docs for bindings you need (for example gtk-3.0) from valadoc.org (the main page; you see a "devhelp-package" link on the right side). Put the extracted folder of this into ~/.local/share/devhelp/books/