docs: Add Korean (한국어) language support for the documentation

[divyanshub024]

Description

The Stac documentation is currently available only in English. Adding Korean (한국어) language support would make the framework more accessible to the Korean developer community, which has a strong and active Flutter ecosystem.

The docs are built with Mintlify, which supports internationalization through the languages / localization configuration in docs.json, so translated pages can live alongside the existing English content with a language switcher in the docs UI.

Requirements

• Configure Korean (ko) as an additional language in docs/docs.json
• Translate the core getting-started pages first (introduction.mdx, quickstart.mdx, sdui.mdx, dsl.mdx, project_structure.mdx, cli.mdx)
• Translate the concepts/ and guides/ sections
• Translate the widgets/ and actions/ reference pages
• Add a language switcher so readers can toggle between English and Korean
• Define a process for keeping translations in sync when English docs change