How do you delete a page in Notability for iPhone?
UPDATE:

I asked Ginger labs and they said:


  Yes, you are correct. You can only delete pages on iPad and Mac. We see how adding this to iPhone would be helpful. We'll add this to the list of possibilities for the future. Thanks for your feedback! 


On the iPad, if you click on the top right icon (the one that looks like pages, not the paintbrush one), you'll see a bunch of thumbnails. (My screenshot is from the Mac app, but the layout is similar.) This is the "page navigator icon".



Now you should see red "x" circles on different pages. Tap those and you can delete a particular page.

Ginger Labs Support: Can I Insert, Delete, or Clear a Page?