Make `i18n-helpers` a requirement for running `mdbook`

[mgeisler]

We can simplify the setup a bit by making the i18n-helpers a mandatory requirement for running mdbook.

I was originally hesitant to complicate the setup instructions, but since people already have to install mdbook-svgbob, I now think it will be fine to also ask them to install i18n-helpers.

We should also think about publishing i18n-helpers to crates.io so that people can install it easily for other projects. That's more of a long-term plan, though.

[mgeisler]

Doing this would remove some warnings triggered by #429 and #421.

[jooyunghan]

Fixed with #461

Publishing it could be another story.