docs(i18n): add Korean translation structure and index page

[1wos]

Title

docs(i18n): add Korean translation structure and index page

Type / Scope

• Type: Docs
• Scope: i18n (Korean)

Summary / Motivation

• Add Korean (ko) translation directory under docs/source/ko/ as discussed in 🌐 [i18n-KO] Translating docs to Korean #3058.
• Includes translated index.mdx, full _toctree.yml, and in_translation.md placeholder.
• Translation follows the HuggingFace Transformers Korean translation conventions:
  established ML terms in Korean (정책, 추론, 모방 학습), loanwords in Hangul (프로세서, 데이터셋), proper nouns in English (ACT, LoRA).

Related issues

• Related: 🌐 [i18n-KO] Translating docs to Korean #3058

What changed

• docs/source/ko/index.mdx — translated landing page
• docs/source/ko/_toctree.yml — full table of contents in Korean
• docs/source/ko/in_translation.md — placeholder for untranslated pages

Questions for maintainers

@pkooij Following the transformers docs structure, English docs live under en/. Should we:

1. Move existing docs into docs/source/en/ to match that multi-language structure?
2. Or keep the current flat structure and only add ko/?

Happy to help with either approach!

How was this tested (or how to run locally)

• Pre-commit checks all passed:

  pre-commit run --files docs/source/ko/_toctree.yml docs/source/ko/in_translation.md docs/source/ko/index.mdx ```