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/docusaurus.config.ts

@@ -24,7 +24,7 @@ const config: Config = {
 
   i18n: {
     defaultLocale: 'en',
-    locales: ['en', 'zh-Hans', 'ko'],
+    locales: ['en', 'zh-Hans'],
     localeConfigs: {
       en: {
         label: 'English',
@@ -33,10 +33,6 @@ const config: Config = {
         label: '简体中文',
         htmlLang: 'zh-Hans',
       },
-      ko: {
-        label: '한국어',
-        htmlLang: 'ko',
-      },
     },
   },