Add a placeholder page when the manual page is not found

The "Learn more..." link on the app page for Kiwix, a new app added in version 23.19, returns a 404 page not found because there's not yet a manual page for Kiwix.

Perhaps all new apps, even those flagged as advanced, should be accompanied by at least a placeholder manual page announcing the app, since a 404 is not a good look.