Restructure documentation. Now the documentation has installation, administration, usage, development, contributing the 5 main parts (#23629)

- **Installation**: includes how to install Gitea and related other
tools, also includes upgrade Gitea
- **Administration**: includes how to configure Gitea, customize Gitea
and manage Gitea instance out of Gitea admin UI
- **Usage**: includes how to use Gitea's functionalities. A sub
documentation is about packages, in future we could also include CI/CD
and others.
- **Development**: includes how to integrate with Gitea's API, how to
develop new features within Gitea
- **Contributing**: includes how to contribute code to Gitea
repositories.

After this is merged, I think we can have a sub-documentation of `Usage`
part named `Actions` to describe how to use Gitea actions

---------

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

docs/content/doc/administration/environment-variables.en-us.md

@@ -0,0 +1,63 @@
+---
+date: "2017-04-08T11:34:00+02:00"
+title: "Environment variables"
+slug: "environment-variables"
+weight: 20
+toc: false
+draft: false
+menu:
+  sidebar:
+    parent: "administration"
+    name: "Environment variables"
+    weight: 10
+    identifier: "environment-variables"
+---
+
+# Environment variables
+
+**Table of Contents**
+
+{{< toc >}}
+
+This is an inventory of Gitea environment variables. They change Gitea behaviour.
+
+Initialize them before Gitea command to be effective, for example:
+
+```sh
+GITEA_CUSTOM=/home/gitea/custom ./gitea web
+```
+
+## From Go language
+
+As Gitea is written in Go, it uses some Go variables, such as:
+
+- `GOOS`
+- `GOARCH`
+- [`GOPATH`](https://golang.org/cmd/go/#hdr-GOPATH_environment_variable)
+
+For documentation about each of the variables available, refer to the
+[official Go documentation](https://golang.org/cmd/go/#hdr-Environment_variables).
+
+## Gitea files
+
+- `GITEA_WORK_DIR`: Absolute path of working directory.
+- `GITEA_CUSTOM`: Gitea uses `GITEA_WORK_DIR`/custom folder by default. Use this variable
+  to change _custom_ directory.
+- `GOGS_WORK_DIR`: Deprecated, use `GITEA_WORK_DIR`
+- `GOGS_CUSTOM`: Deprecated, use `GITEA_CUSTOM`
+
+## Operating system specifics
+
+- `USER`: System user that Gitea will run as. Used for some repository access strings.
+- `USERNAME`: if no `USER` found, Gitea will use `USERNAME`
+- `HOME`: User home directory path. The `USERPROFILE` environment variable is used in Windows.
+
+### Only on Windows
+
+- `USERPROFILE`: User home directory path. If empty, uses `HOMEDRIVE` + `HOMEPATH`
+- `HOMEDRIVE`: Main drive path used to access the home directory (C:)
+- `HOMEPATH`: Home relative path in the given home drive path
+
+## Miscellaneous
+
+- `SKIP_MINWINSVC`: If set to 1, do not run as a service on Windows.