gtk: Change gtk+ documentation URL to correct one since "/documentation.php" returns 404
//!
//! - [General `GLib` family types and object system overview](../glib/index.html)
//!
-//! - [GTK+ documentation](http://www.gtk.org/documentation.php)
+//! - [GTK+ documentation](https://www.gtk.org/docs/)
//!
//! # Hello World
//!
