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/secrets.en-us.md

@@ -0,0 +1,39 @@
+---
+date: "2022-12-19T21:26:00+08:00"
+title: "Secrets"
+slug: "secrets"
+sidebar_position: 50
+draft: false
+toc: false
+aliases:
+  - /en-us/secrets
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Secrets"
+    sidebar_position: 50
+    identifier: "usage-secrets"
+---
+
+# Secrets
+
+Secrets allow you to store sensitive information in your user, organization or repository.
+Secrets are available on Gitea 1.19+ and are only visible in 1.20+ when ACTIONS are enabled.
+
+# Naming your secrets
+
+The following rules apply to secret names:
+
+- Secret names can only contain alphanumeric characters (`[a-z]`, `[A-Z]`, `[0-9]`) or underscores (`_`). Spaces are not allowed.
+
+- Secret names must not start with the `GITHUB_` and `GITEA_` prefix.
+
+- Secret names must not start with a number.
+
+- Secret names are not case-sensitive.
+
+- Secret names must be unique at the level they are created at.
+
+For example, a secret created at the repository level must have a unique name in that repository, and a secret created at the organization level must have a unique name at that level.
+
+If a secret with the same name exists at multiple levels, the secret at the lowest level takes precedence. For example, if an organization-level secret has the same name as a repository-level secret, then the repository-level secret takes precedence.