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/doc/usage/actions/overview.en-us.md

@@ -1,55 +0,0 @@
----
-date: "2023-04-27T15:00:00+08:00"
-title: "Gitea Actions"
-slug: "overview"
-weight: 1
-draft: false
-toc: false
-menu:
-  sidebar:
-    parent: "actions"
-    name: "Overview"
-    weight: 1
-    identifier: "actions-overview"
----
-
-# Gitea Actions
-
-Starting with Gitea **1.19**, Gitea Actions are available as a built-in CI/CD solution.
-
-**Table of Contents**
-
-{{< toc >}}
-
-## Name
-
-It is similar and compatible to [GitHub Actions](https://github.com/features/actions), and its name is inspired by it too.
-To avoid confusion, we have clarified the spelling here:
-
-- "Gitea Actions" (with an "s", both words capitalized) is the name of the Gitea feature.
-- "GitHub Actions" is the name of the GitHub feature.
-- "Actions" could refer to either of the above, depending on the context. So it refers to "Gitea Actions" in this document.
-- "action" or "actions" refer to some scripts/plugins to be used, like "actions/checkout@v3" or "actions/cache@v3".
-
-## Runners
-
-Just like other CI/CD solutions, Gitea doesn't run the jobs itself, but delegates the jobs to runners.
-The runner of Gitea Actions is called [act runner](https://gitea.com/gitea/act_runner), it is a standalone program and also written in Go.
-It is based on a [fork](https://gitea.com/gitea/act) of [nektos/act](http://github.com/nektos/act).
-
-Because the runner is deployed independently, there could be potential security issues.
-To avoid them, please follow two simple rules:
-
-- Don't use a runner you don't trust for your repository, organization or instance.
-- Don't provide a runner to a repository, organization or instance you don't trust.
-
-For Gitea instances used internally, such as instances used by enterprises or individuals, neither of these two rules is a problem, they are naturally so.
-However, for public Gitea instances, such as [gitea.com](https://gitea.com), these two rules should be kept in mind when adding or using runners.
-
-## Status
-
-Gitea Actions is still under development, so there may be some bugs and missing features.
-And breaking changes may be made before it's stable (v1.20 or later).
-
-If the situation changes, we will update it here.
-So please refer to the content here when you find outdated articles elsewhere.