fix(website): cross-locale doc links + drop empty ko locale (#31895)

The locale switcher appeared broken because hardcoded markdown links
(`](/docs/X)`) got double-prefixed by Docusaurus to `/docs/<locale>/docs/X`
(404) in non-English locales, and the MDX hero `<a href>` on the index
page escaped locale routing entirely.

Changes:
- Rewrite 922 `](/docs/X)` -> `](/X)` across 166 docs files (strip trailing
  .md too). Docusaurus prepends locale + baseUrl itself.
- docs/index.md -> index.mdx; hero "Get Started" anchor -> Docusaurus
  <Link> so it stays inside the active locale.
- Drop `ko` locale entirely from docusaurus.config.ts + delete i18n/ko/
  (4 stale auto-translated kanban pages, <2% coverage, misleading).

Verified `npm run build` succeeds for both en and zh-Hans; `build/zh-Hans/
index.html` has no /docs/zh-Hans/docs/... double-prefixed paths.

PR2 will translate the 335 English docs into i18n/zh-Hans/.

website/docs/developer-guide/context-engine-plugin.md

@@ -189,6 +189,6 @@ See `tests/agent/test_context_engine.py` for the full ABC contract test suite.
 
 ## See also
 
-- [Context Compression and Caching](/docs/developer-guide/context-compression-and-caching) — how the built-in compressor works
-- [Memory Provider Plugins](/docs/developer-guide/memory-provider-plugin) — analogous single-select plugin system for memory
-- [Plugins](/docs/user-guide/features/plugins) — general plugin system overview
+- [Context Compression and Caching](/developer-guide/context-compression-and-caching) — how the built-in compressor works
+- [Memory Provider Plugins](/developer-guide/memory-provider-plugin) — analogous single-select plugin system for memory
+- [Plugins](/user-guide/features/plugins) — general plugin system overview