Add parallax effect (#170)

Creates a parallax scrolling effect.

Co-authored-by: Jonathan Milanovic <[email protected]>

Author: floatdrop

src/styles/blocks/header.styl

@@ -1,6 +1,6 @@
 .header
   text-align center
-  background #222 url('/assets/transparent.jpg') center / cover
+  background #222 url('/assets/transparent.jpg') no-repeat center / cover fixed
   img
     margin 32px 0
     width 256px