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-1.26.json

@@ -0,0 +1,29 @@
+{
+  "sidebar.docs.category.Installation": {
+    "message": "安裝"
+  },
+  "sidebar.docs.category.Administration": {
+    "message": "管理"
+  },
+  "sidebar.docs.category.Usage": {
+    "message": "使用"
+  },
+  "sidebar.docs.category.actions": {
+    "message": "Actions"
+  },
+  "sidebar.docs.category.packages": {
+    "message": "套件"
+  },
+  "sidebar.docs.category.Development": {
+    "message": "開發"
+  },
+  "sidebar.docs.category.Contributing": {
+    "message": "貢獻"
+  },
+  "sidebar.docs.category.Help": {
+    "message": "幫助"
+  },
+  "sidebar.docs.doc.What is Gitea?": {
+    "message": "Gitea是什麼?"
+  }
+}