Fix soft 404

[nineteendo]

https://support.google.com/webmasters/answer/7440203#soft_404:

The page request returns what we think is a soft 404 response. This means that it returns a user-friendly "not found" message but not a 404 HTTP response code. We recommend returning a 404 response code for truly "not found" pages and adding more information on the page to let us know that it is not a soft 404. To see how Google sees the page, run a live URL inspection test against the page and click View tested page to see a screenshot showing how Google renders the page. Learn how to fix a soft 404.

https://sphinx-notfound-page.readthedocs.io/en/latest/404.html: