https://api.drupal.org/api/drupal/elements/8.2.x
Agregar Agrupación
$form[galery] = array(
#type => details,
#title => $this->t(Datos de Galeria),
);
$form[galery][number_albums] = [
#type => number,
#title => $this->t(Cantidad de albunes),
#description => $this->t(Muestra la cantidad de albums que se mostrar),
#maxlength => 10,
#size => 15,
#default_value => $config->get(number_albums),
];