Finally added a page for the GTK inspector on the GNOME developers docs website: https://developer.gnome.org/documentation/tools/inspector.html
This way, I won't have to point people at the wiki any more.
GTK Inspector
The GTK inspector is the built-in interactive debugging support in GTK. The Inspector is extremely powerful, and allows designers and application developers to test CSS changes on-the-fly, magnify ...GNOME Developer Documentation