🌐 [i18n-KO] Translated `bonus-unit3/introduction.mdx` to Korean

[iambogeumkim]

What does this PR do?

Translated the bonus-unit3/introduction.mdx file of the documentation to Korean. Thank you in advance for your review.

Before reviewing

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

Who can review? (Initial)

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

Before submitting

• [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] 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.
• [x] 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)