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/user-guide/skills/optional/finance/finance-comps-analysis.md

@@ -21,7 +21,7 @@ Build comparable company analysis in Excel — operating metrics, valuation mult
 | License | Apache-2.0 |
 | Platforms | linux, macos, windows |
 | Tags | `finance`, `valuation`, `comps`, `excel`, `openpyxl`, `modeling`, `investment-banking` |
-| Related skills | [`excel-author`](/docs/user-guide/skills/optional/finance/finance-excel-author), [`pptx-author`](/docs/user-guide/skills/optional/finance/finance-pptx-author), [`dcf-model`](/docs/user-guide/skills/optional/finance/finance-dcf-model), [`lbo-model`](/docs/user-guide/skills/optional/finance/finance-lbo-model) |
+| Related skills | [`excel-author`](/user-guide/skills/optional/finance/finance-excel-author), [`pptx-author`](/user-guide/skills/optional/finance/finance-pptx-author), [`dcf-model`](/user-guide/skills/optional/finance/finance-dcf-model), [`lbo-model`](/user-guide/skills/optional/finance/finance-lbo-model) |
 
 ## Reference: full SKILL.md