Add next gen image format for background

This apparently helps with page loads and in turn SEO

Author: demophoon

assets/static/backgrounds/online.jpf

@@ -7,7 +7,7 @@ block main
 
     section#intro
         .background(du-parallax, y="animator")
-            .image.bg-cover(ff-bg="static/backgrounds/online.jpg")
+            .image.bg-cover(ff-bg="static/backgrounds/online.jpf")
         .image-attribution
             | Header photo © #[a(href="https://thea.codes") Alethea Flowers] / CC BY-SA 4.0
         .overlay