docs: port to gi-docgen
As mentioned on the title, this PR includes a set of changes to port the docs away from gtk-doc to gi-docgen.
The docs can be viewed in https://gnome.pages.gitlab.gnome.org/-/geocode-glib/-/jobs/1736588/artifacts/_reference/geocode-glib-1/index.html
Edited by Bilal Elmoussaoui