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/zh-tw/docs/version-1.22/installation.md

@@ -0,0 +1,12 @@
+---
+date: "2016-12-01T16:00:00+02:00"
+title: "安裝"
+slug: "installation"
+sidebar_position: 10
+
+menu:
+  sidebar:
+    name: "安裝"
+    sidebar_position: 10
+    identifier: "installation"
+---