docs: add Korean Kanban documentation

[pochi-gio]

Summary

• add a Korean locale to the docs site
• add Korean translations for the Kanban overview and tutorial
• add Korean translations for the bundled kanban-orchestrator and kanban-worker skill docs

Why

There wasn't an existing Korean translation for the Kanban docs/skills, so this PR adds the first-pass localization for the Kanban documentation set.

Testing

• cd website && npm ci
• cd website && npm run build
  • build succeeds for en, zh-Hans, and ko
  • existing unrelated broken-link / broken-anchor warnings still appear in the repository's translated docs and one pre-existing English docs page

Notes

• I added ko as a Docusaurus locale.
• I intentionally did not add ko to the local-search plugin language list because @easyops-cn/docusaurus-search-local currently errors on Korean regex generation during build.
• Updated the translated Kanban links to stay locale-aware (relative links instead of /docs/... absolute paths).

Platforms tested

• Linux