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/glib/meson.build

@@ -88,7 +88,8 @@ if get_option('gtk_doc')
                fixxref_args: [
                  '--html-dir=' + docpath,
                ],
-               install: true)
+               install: true,
+               check: true)
 endif
 
 if get_option('man')