docs: add Korean Kanban documentation
Salvages #21823 by @pochi-gio. Adds Korean (ko) Docusaurus locale and
translates Kanban documentation (kanban.md, kanban-tutorial.md) and the
two related skills (devops-kanban-orchestrator, devops-kanban-worker).
Purely additive — adds ko to the locales list in docusaurus.config.ts
and creates the website/i18n/ko/ tree.