$entity_types['node']
->setFormClass('book_outline', 'Drupal\book\Form\BookOutlineForm')
->setLinkTemplate('book-outline-form', '/node/{node}/outline')
$entity_types['node']
->setFormClass('book_outline', 'Drupal\book\Form\BookOutlineForm')
->setLinkTemplate('book-outline-form', '/node/{node}/outline')