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/code-owners.en-us.md

@@ -0,0 +1,65 @@
+---
+date: "2023-05-24T16:00:00+00:00"
+title: "Code Owners"
+slug: "code-owners"
+sidebar_position: 30
+toc: false
+draft: false
+aliases:
+  - /en-us/code-owners
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Code Owners"
+    sidebar_position: 30
+    identifier: "code-owners"
+---
+
+# Code Owners
+
+Gitea maintains code owner files. It looks for it in the following locations in this order:
+
+- `./CODEOWNERS`
+- `./docs/CODEOWNERS`
+- `./.gitea/CODEOWNERS`
+
+And stops at the first found file.
+
+File format: `<regexp rule> <@user or @org/team> [@user or @org/team]...`
+
+Regexp specified in golang Regex format.
+Regexp can start with `!` for negative rules - match all files except specified.
+
+Example file:
+
+```
+.*\\.go @user1 @user2 # This is comment
+
+# Comment too
+# You can assigning code owning for users or teams
+frontend/src/.*\\.js @org1/team1 @org1/team2 @user3
+
+# You can use negative pattern
+!frontend/src/.* @org1/team3 @user5
+
+# You can use power of go regexp
+docs/(aws|google|azure)/[^/]*\\.(md|txt) @user8 @org1/team4
+!/assets/.*\\.(bin|exe|msi) @user9
+```
+
+### Escaping
+
+You can escape characters `#`, ` ` (space) and `\` with `\`, like:
+
+```
+dir/with\#hashtag @user1
+path\ with\ space @user2
+path/with\\backslash @user3
+```
+
+Some character (`.+*?()|[]{}^$\`) should be escaped with `\\` inside regexp, like:
+
+```
+path/\\.with\\.dots
+path/with\\+plus
+```