Link to content within node, from CKEditor WYSIWYG
There's an option, under "Url Substitution" for linkit Media matchers that allows exactly this, but for Media entities. It should be fairly straightforward to create a new plugin using Drupal\linkit\Plugin\Linkit\Substitution\Media as a reference. (You should just have to update the plugin definition annotation as well as the getUrl() and isApplicable() methods. Then you can exclude your Document content type from any previous linkit matcher and add a new linkit matcher to your profile that uses your new plugin for Url Substitution type to link to Documents.