move 404 page to content

see "Option 2" in https://moonbooth.com/hugo/custom-404/

Author: danielpeintner

content/404.md

@@ -0,0 +1,9 @@
+---
+title: Whoops! Page not found
+noindex: true
+layout: page
+---
+
+This page can't be found.
+
+Try to find the content on our [homepage](/).