Docusaurus-ify (#26051)

This PR cleans up the docs in a way to make them simpler to ingest by
our [docs repo](https://gitea.com/gitea/gitea-docusaurus).

1. It includes all of the sed invocations our ingestion did, removing
the need to do it at build time.
2. It replaces the shortcode variable replacement method with
`@variable@` style, simply for easier sed invocations when required.
3. It removes unused files and moves the docs up a level as cleanup.

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

docs/content/usage/authentication.zh-tw.md

@@ -0,0 +1,20 @@
+---
+date: "2016-12-01T16:00:00+02:00"
+title: "認證"
+slug: "authentication"
+sidebar_position: 10
+toc: false
+draft: false
+aliases:
+  - /zh-tw/authentication
+menu:
+  sidebar:
+    parent: "usage"
+    name: "認證"
+    sidebar_position: 10
+    identifier: "authentication"
+---
+
+# 認證
+
+## TBD