Notice in that section on Overriding… that they show the directory as being myproject and not myapp for those files. I would interpret that as meaning that no - a custom admin site that is replacing the default does not belong in an app, it’s a project-level configuration item.
Your other apps then would import the site from myproject.admin and not core.admin.
(This is under the working assumption that you’re using the default directory structure that includes a myproject directory that is inside the BASE_DIR myproject directory.)