Nesting module-defined menu links in Drupal 8
When you provide a module-defined menu link in Drupal 8, there is some great documentation on how to add a menu link on Drupal.org. This gets into how to provide a menu link with YAML. In a lot of cases, you might want to nest this menu link under another item. This is especially the case if you were providing a menu link for something in the main menu of your site.