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/contributing.en-us.md

@@ -0,0 +1,13 @@
+---
+date: "2021-01-22T00:00:00+02:00"
+title: "Contributing"
+slug: "contributing"
+sidebar_position: 35
+toc: false
+draft: false
+menu:
+  sidebar:
+    name: "Contributing"
+    sidebar_position: 50
+    identifier: "contributing"
+---