gplugin/gplugin

c3f82c2ae491
Move to using gi-docgen from a release rather than git

Testing Done:
Deleted the checked out copies in `subprojects` and did a build with the docs turned on.

Reviewed at https://reviews.imfreedom.org/r/2271/
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Paul Jimenez <pj@place.org>
Richard Laager <rlaager@pidgin.im>
Sorvival <sorvival@tutanota.com>
ivanhoe <ivanhoe@fiscari.de>