docs(ko): rename “Get Started” → “시작하기” and polish nav‑card texts

[Gaic4o]

Background

To keep the Korean docs fully localised, we changed the page header from “Get Started” to “시작하기” and rewrote the summary / NavCard descriptions for clearer, action‑oriented wording.
Action‑verbs (“살펴봅니다”, “적용해 봅니다”, “해결합니다”) guide readers toward the next step and improve scan‑ability, especially for first‑time visitors.

Changelog

• Rename page heading: # 🚀 Get Started → # 🚀 시작하기.
• Rephrased summary paragraph (shorter, highlights benefits and background).
• Updated NavCard descriptions:
  • Overview – “FSD’s concept, structure, usage at a glance.”
  • Tutorial – “Hands‑on steps to apply FSD.”
  • FAQ – “Answers to common questions.”