page_example.links.menu.yml

page_example.description:
  title: Page Example
  route_name: page_example.description
  expanded: TRUE

page_example.simple:
  title: Simple - no arguments
  route_name: page_example.simple
  parent: page_example.description

# We can't define a menu link for the page_example_arguments route, because it
# requires path arguments.