The follow-up effort to this can be found at https://github.com/kugel-/geany/tree/doxygen2gtkdoc
There is the script to generate the gtkdoc and to create a .vapi, .gir and .typelib automatically for the plugin API.
--- Reply to this email directly or view it on GitHub: https://github.com/geany/geany/pull/744#issuecomment-155109314