Add option for deleting pages after the the image/PDF is generated

[jerboa88]

If possible, use the Gatby's deletePage() action to delete pages after the relevant images/PDFs have been generated.

If the user doesn't plan to use the generated pages for any other purpose, removing them can reduce the build size and eliminate the need to manually exclude these pages from other plugins like gatsby-plugin-sitemap.

Consider removing pages by default.