Restructure documentation. Now the documentation has installation, administration, usage, development, contributing the 5 main parts (#23629)

- **Installation**: includes how to install Gitea and related other
tools, also includes upgrade Gitea
- **Administration**: includes how to configure Gitea, customize Gitea
and manage Gitea instance out of Gitea admin UI
- **Usage**: includes how to use Gitea's functionalities. A sub
documentation is about packages, in future we could also include CI/CD
and others.
- **Development**: includes how to integrate with Gitea's API, how to
develop new features within Gitea
- **Contributing**: includes how to contribute code to Gitea
repositories.

After this is merged, I think we can have a sub-documentation of `Usage`
part named `Actions` to describe how to use Gitea actions

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

docs/content/doc/developers/migrations.zh-tw.md

@@ -1,39 +0,0 @@
----
-date: "2019-04-15T17:29:00+08:00"
-title: "遷移介面"
-slug: "migrations-interfaces"
-weight: 30
-toc: false
-draft: false
-menu:
-  sidebar:
-    parent: "developers"
-    name: "遷移介面"
-    weight: 55
-    identifier: "migrations-interfaces"
----
-
-# 遷移功能
-
-完整的遷移從 Gitea 1.9.0 開始提供。它定義了兩個介面用來從其它 Git 託管平臺遷移儲存庫資料到 Gitea，未來或許會提供遷移到其它 git 託管平臺。
-目前已實作了從 Github, Gitlab 和其它 Gitea 遷移資料。
-
-Gitea 定義了一些基本物件於套件 [modules/migration](https://github.com/go-gitea/gitea/tree/master/modules/migration)。
-分別是 `Repository`, `Milestone`, `Release`, `ReleaseAsset`, `Label`, `Issue`, `Comment`, `PullRequest`, `Reaction`, `Review`, `ReviewComment`。
-
-## Downloader 介面
-
-從新的 Git 託管平臺遷移，有兩個新的步驟。
-
-- 您必須實作一個 `Downloader`，它用來取得儲存庫資訊。
-- 您必須實作一個 `DownloaderFactory`，它用來偵測 URL 是否符合並建立上述的 `Downloader`。
-  - 您需要在 `init()` 透過 `RegisterDownloaderFactory` 來註冊 `DownloaderFactory`。
-
-您可以在 [downloader.go](https://github.com/go-gitea/gitea/blob/master/modules/migration/downloader.go) 中找到這些介面。
-
-## Uploader 介面
-
-目前只有 `GiteaLocalUploader` 被實作出來，所以我們只能通過 `Uploader` 儲存已下載的資料到本地的 Gitea 實例。
-目前尚未支援其它 Uploader。
-
-您可以在 [uploader.go](https://github.com/go-gitea/gitea/blob/master/modules/migration/uploader.go) 中找到這些介面。