Custom 404 page
In order to set up a custom 404 page, please add a file called 404.html to
your site. This file needs to be in the public folder when deploying.
Note that this also applies to pages for unauthorized requests (if you have a
password-protected site). In this case, please add a file called 401.html to
your site (401 stands for the HTTP status code “401 Unauthorized”)