doc: Run gtkdoc-check with unit tests

This has the side effect of always rebuilding the doc at each build when
gtk_doc option is enabled (not by default). Most importantly, this will
enable doc check on our CI.

docs/reference/gio/meson.build

@@ -209,7 +209,8 @@ if get_option('gtk_doc')
       '--extra-dir=' + join_paths('gio', '..', 'glib', 'html'),
       '--extra-dir=' + join_paths('gio', '..', 'gobject', 'html'),
     ],
-    install: true
+    install: true,
+    check: true,
   )
 endif