Add 404 error page and update book configuration

[yl275]

This PR adds a small JavaScript snippet to 404.md, so that when a user visits a broken link under a translated version of the book (for example /es/does-not-exist.html), they are redirected to the corresponding language-specific 404 page (/es/404.html, /zh-CN/404.html, etc.) instead of the English 404.

I tested this, but it seems that 404.md is not going through the translation pipeline.
Am I on the right track with this approach?

fixes #2022

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[mgeisler]

I tested this, but it seems that 404.md is not going through the translation pipeline.

I haven't tested this myself, but what you write makes sense: we extract text from the book "chapters", which are the files mentioned in SUMMARY.md. So the 404 is not there, so I would not expect the text to be extracted.

To improve upon this, I think we would need to add code in mdbook-i18n-helpers to detect the input-404 config setting (I hope we have access to the book.toml data there?) and extract the data manually.

We would then also need to test if the page can be translated later...

Am I on the right track with this approach?

Yes, I think this looks like a great approach!

I don't have a lot of time to really test this out right now, but it looks like a nice approach to me.