Add configuration reference

docs/ref/configuration.md

@@ -0,0 +1,39 @@
+# Configuration
+
+- Headscale loads its configuration from a YAML file
+- It searches for `config.yaml` in the following paths:
+  - `/etc/headscale`
+  - `$HOME/.headscale`
+  - the current working directory
+- Use the command line flag `-c`, `--config` to load the configuration from a different path
+- Validate the configuration file with: `headscale configtest`
+
+!!! example "Get the [example configuration from the GitHub repository](https://github.com/juanfont/headscale/blob/main/config-example.yaml)"
+
+    Always select the [same GitHub tag](https://github.com/juanfont/headscale/tags) as the released version you use to
+    ensure you have the correct example configuration. The `main` branch might contain unreleased changes.
+
+    === "View on GitHub"
+
+        * Development version: <https://github.com/juanfont/headscale/blob/main/config-example.yaml>
+        * Version 0.23.0: <https://github.com/juanfont/headscale/blob/v0.23.0/config-example.yaml>
+
+    === "Download with `wget`"
+
+        ```shell
+        # Development version
+        wget -O config.yaml https://raw.githubusercontent.com/juanfont/headscale/main/config-example.yaml
+
+        # Version 0.23.0
+        wget -O config.yaml https://raw.githubusercontent.com/juanfont/headscale/v0.23.0/config-example.yaml
+        ```
+
+    === "Download with `curl`"
+
+        ```shell
+        # Development version
+        curl -o config.yaml https://raw.githubusercontent.com/juanfont/headscale/main/config-example.yaml
+
+        # Version 0.23.0
+        curl -o config.yaml https://raw.githubusercontent.com/juanfont/headscale/v0.23.0/config-example.yaml
+        ```