collection-8.x-1.x-dev/collection.links.menu.yml

entity.collection.collection:
  title: 'Collections'
  route_name: entity.collection.collection
  description: 'Manage Collections'
  parent: system.admin

# Collection type menu items definition
entity.collection_type.collection:
  title: 'Collection types'
  route_name: entity.collection_type.collection
  description: 'Manage collection types'
  parent: system.admin_structure

# Collection item type menu items definition
entity.collection_item_type.collection:
  title: 'Collection item types'
  route_name: entity.collection_item_type.collection
  description: 'Manage collection item types'
  parent: system.admin_structure