refactor: rename i18n/*/docusaurus-plugin-content-docs to i18n/*/docs (#588)

Drops the leftover docusaurus name from the translated content directories.

- git mv i18n/{en-us,zh-cn,zh-tw}/docusaurus-plugin-content-docs -> i18n/{locale}/docs (pure renames, content unchanged)
- update the paths in packages/content-loader/src/products.ts, scripts/cut-version.mjs and sites/docs/README.md

current.json / version-*.json keep their names and stay beside the content.

Verified: pnpm build succeeds, 6690 pages built.

<!-- cloudflare-preview --> Preview: https://pr-588.docs-gitea-com.pages.dev
Reviewed-on: https://gitea.com/gitea/docs/pulls/588
Reviewed-by: Zettat123 <[email protected]>

i18n/en-us/docs/current.json

@@ -0,0 +1,8 @@
+{
+  "sidebar.docs.category.actions": {
+    "message": "Actions"
+  },
+  "sidebar.docs.category.packages": {
+    "message": "Packages"
+  }
+}