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/development/migrations.en-us.md

@@ -0,0 +1,43 @@
+---
+date: "2019-04-15T17:29:00+08:00"
+title: "Migrations Interfaces"
+slug: "migrations-interfaces"
+sidebar_position: 55
+toc: false
+draft: false
+aliases:
+  - /en-us/migrations-interfaces
+menu:
+  sidebar:
+    parent: "development"
+    name: "Migrations Interfaces"
+    sidebar_position: 55
+    identifier: "migrations-interfaces"
+---
+
+# Migration Features
+
+Complete migrations were introduced in Gitea 1.9.0. It defines two interfaces to support migrating
+repository data from other Git host platforms to Gitea or, in the future, migrating Gitea data to other Git host platforms.
+
+Currently, migrations from GitHub, GitLab, and other Gitea instances are implemented.
+
+First of all, Gitea defines some standard objects in packages [modules/migration](https://github.com/go-gitea/gitea/tree/main/modules/migration).
+They are `Repository`, `Milestone`, `Release`, `ReleaseAsset`, `Label`, `Issue`, `Comment`, `PullRequest`, `Reaction`, `Review`, `ReviewComment`.
+
+## Downloader Interfaces
+
+To migrate from a new Git host platform, there are two steps to be updated.
+
+- You should implement a `Downloader` which will be used to get repository information.
+- You should implement a `DownloaderFactory` which will be used to detect if the URL matches and create the above `Downloader`.
+  - You'll need to register the `DownloaderFactory` via `RegisterDownloaderFactory` on `init()`.
+
+You can find these interfaces in [downloader.go](https://github.com/go-gitea/gitea/blob/main/modules/migration/downloader.go).
+
+## Uploader Interface
+
+Currently, only a `GiteaLocalUploader` is implemented, so we only save downloaded
+data via this `Uploader` to the local Gitea instance. Other uploaders are not supported at this time.
+
+You can find these interfaces in [uploader.go](https://github.com/go-gitea/gitea/blob/main/modules/migration/uploader.go).