Move menu page to the Pods Admin menu.

@pglewis I'm mainly doing this to test that this method works, to resolve https://github.com/pods-framework/pods/issues/2079 . Feel free to disregard. Results are inconclusive as the menu page is moved, but renders with header only. I reverted my changes, and saw that the admin page was empty without the changes as well, so pretty sure I didn't break it.