Use 404 and 410 pages of whatwg.org elsewhere

[annevk]

html.spec.whatwg.org needs a custom 404, but otherwise we could reuse https://whatwg.org/404 and https://whatwg.org/410 across our domains as they are much nicer than the defaults.

This would require changing https://github.com/whatwg/whatwg.org/blob/main/whatwg.org/404 and https://github.com/whatwg/whatwg.org/blob/main/whatwg.org/410 to allow them to be served from other domains than whatwg.org.

I'm happy to do this work, but I need some guidance on how to change the server configuration. In particular it would be nice if we didn't have to duplicate these files.