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/README.md

@@ -3,36 +3,7 @@
 [![Join the chat at https://img.shields.io/discord/322538954119184384.svg](https://img.shields.io/discord/322538954119184384.svg)](https://discord.gg/Gitea)
 [![](https://images.microbadger.com/badges/image/gitea/docs.svg)](http://microbadger.com/images/gitea/docs "Get your own image badge on microbadger.com")
 
-## Hosting
-
-These pages are hosted using [netlifycms](https://www.netlifycms.org/) and get
-automatically updated on every push to the `master` branch.
-
-## Install
-
-These pages use the [Hugo](https://gohugo.io/) static site generator.
-If you are planning to contribute you'll want to download and install Hugo on
-your local machine.
-
-The installation of Hugo is out of the scope of this document, so please take
-the [official install instructions](https://gohugo.io/installation/) to
-get Hugo up and running.
-
-## Development
-
-To generate the website and serve it on [localhost:1313](http://localhost:1313)
-just execute this command and stop it with `Ctrl+C`:
-
-```
-make server
-```
-
-When you are done with your changes just create a pull request, after merging
-the pull request the website will be updated automatically.
-
-## Contributing
-
-Fork -> Patch -> Push -> Pull Request
+These docs are ingested by our [docs repo](https://gitea.com/gitea/gitea-docusaurus).
 
 ## Authors