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/architecture.md

@@ -231,7 +231,7 @@ Long-running process with 20 platform adapters, unified session routing, user au
 
 Three discovery sources: `~/.hermes/plugins/` (user), `.hermes/plugins/` (project), and pip entry points. Plugins register tools, hooks, and CLI commands through a context API. Two specialized plugin types exist: memory providers (`plugins/memory/`) and context engines (`plugins/context_engine/`). Both are single-select — only one of each can be active at a time, configured via `hermes plugins` or `config.yaml`.
 
-→ [Plugin Guide](/docs/guides/build-a-hermes-plugin), [Memory Provider Plugin](./memory-provider-plugin.md)
+→ [Plugin Guide](/guides/build-a-hermes-plugin), [Memory Provider Plugin](./memory-provider-plugin.md)
 
 ### Cron