Tenth Korean l10n work for release 1.18

- Translate concepts/extend-kubenetes/service-catalog.md in Korean (#23233)
- Translate concepts/configuration/secret.md into Korean (#23110)
- Translate tasks/extend-kubernetes/setup-extension-api-server.md into … (#22574)
- Update outdated files in dev-1.18-ko.10 branch (#23013)

Co-authored-by: Seokho Son <shsongist@gmail.com>
Co-authored-by: Jerry Park <jaehwa@gmail.com>
Co-authored-by: santachopa <santachopa@naver.com>

content/ko/docs/tasks/extend-kubernetes/_index.md

@@ -0,0 +1,5 @@
+---
+title: "쿠버네티스 확장"
+description: 쿠버네티스 클러스터를 작업 환경의 요구에 맞게 조정하는 고급 과정을 이해한다.
+weight: 90
+---