Add multilingual documentation structure for mdBook

Currently, the documentation is written in English and built with mdBook. It would be very helpful if the docs were structured in a way that supports multiple languages (for example, using `translations` directories or mdBook’s [[multilingual book setups](https://rust-lang.github.io/mdBook/format/summary.html#multilingual-books)](https://rust-lang.github.io/mdBook/format/summary.html#multilingual-books)).

I’m not very familiar with setting up this structure myself, but once the framework for multilingual docs is in place, I would be happy to contribute a full Korean translation right away.

This would make Candle’s documentation more accessible to non-English speakers and help grow the community further.

Thanks for considering this!

Start by reviewing the current mdBook documentation setup and mdBook’s multilingual book setup guidance, including the suggested translations-directory approach. Done means the documentation has a clear structure that supports multiple languages and provides a practical path for adding the proposed Korean translation.

Written by the indexing model from the issue text.