🌐 [i18n-KO] Translated unit4 introduction.mdx to Korean

[Youngdong2]

What does this PR do?

Translated the unit4 introduction.mdx file of the documentation to Korean.
Thank you in advance for your review.

Part of huggingface/transformers#20179

Before reviewing

• Check for missing / redundant translations (번역 누락/중복 검사)
• Grammar Check (맞춤법 검사)
• Review or Add new terms to glossary (용어 확인 및 추가)
• Check Inline TOC (e.g. [[lowercased-header]])
• Check live-preview for gotchas (live-preview로 정상작동 확인)

Who can review? (Initial)

May you please review this PR?
@harheem, @nsbg, @Youngdong2, @xhaktm00, @Judy-Choi

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review? (Final)

[xhaktm00]

전체적으로 자연스러운 번역인 것 같습니다. 고생 많으셨습니다!
다만, 기존 번역에서 Unit 용어를 번역하지 않고 그대로 사용했어서, ‘유닛’ 대신 ‘Unit’ 표기 유지 제안드립니다. 확인 부탁드립니다!

[harheem]

수정님의 제안 사항 반영하면 Approve 드리겠습니다 👍🏻

[Youngdong2]

@harheem 수정님 제안사항 반영했습니다.
너무 늦었네요ㅠㅠ 죄송합니다

[sergiopaniego]

Hi @Youngdong2
Is the PR ready for review? You can change the state if so 😄