Add error_page to docs config (#637)

Author: StanFromIreland

salt/docs/config/nginx.docs-redirects.conf

@@ -188,9 +188,11 @@ location ~ ^/((2|3)(\.[0-9]+)?|dev)/\w+/[\d\w\.]+(?!\.html)$ {
 # Add the Surrogate-Key for bulk CDN purging
 # xref https://docs.fastly.com/en/guides/working-with-surrogate-keys
 location ~ ^/((2|3)(\.[0-9]+)?|dev)/ {
+    error_page 404 /$1/404.html;
     add_header Surrogate-Key en/$1 always;
 }
 location ~ ^/(bn-in|el|es|fr|id|it|ja|ko|pl|pt-br|ro|tr|sv|uk|zh-cn|zh-tw)/((2|3)(\.[0-9]+)?|dev)/ {
+    error_page 404 /$1/$2/404.html;
     add_header Surrogate-Key $1/$2 always;
 }