docs: Move the GIOError SECTION

Move it to a separate page as there’s no struct documentation to hang it
off.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/gio/meson.build

@@ -230,6 +230,7 @@ expand_content_files = [
   'dbus-name-owning.md',
   'dbus-name-watching.md',
   'dbus-utils.md',
+  'error.md',
   'file-attributes.md',
   'migrating-gconf.md',
   'migrating-gdbus.md',