Describe how to remove a DERP region

Add documentation for d29feaef.

Fixes: #2450

derp-example.yaml

@@ -1,5 +1,6 @@
 # If you plan to somehow use headscale, please deploy your own DERP infra: https://tailscale.com/kb/1118/custom-derp-servers/
 regions:
+  1: null   # Disable DERP region with ID 1
   900:
     regionid: 900
     regioncode: custom

docs/ref/derp.md

@@ -54,11 +54,33 @@ derp:
 ### Customize DERP map
 
 The DERP map offered to clients can be customized with a [dedicated YAML-configuration
-file](https://github.com/juanfont/headscale/blob/main/derp-example.yaml). Typical use-cases involve:
+file](https://github.com/juanfont/headscale/blob/main/derp-example.yaml). This allows to modify previously loaded DERP
+maps fetched via URL or to offer your own, custom DERP servers to nodes.
 
-- Running a fleet of [custom DERP servers](https://tailscale.com/kb/1118/custom-derp-servers)
+=== "Remove specific DERP regions"
-- Excluding or choosing specific regions from the Tailscale's list of free-to-use [DERP
+
-  servers](https://tailscale.com/kb/1232/derp-servers)
+    The free-to-use [DERP servers](https://tailscale.com/kb/1232/derp-servers) are organized into regions via a region
+    ID. You can explicitly disable a specific region by setting its region ID to `null`. The following sample
+    `derp.yaml` disables the New York DERP region (which has the region ID 1):
+
+     ```yaml title="derp.yaml"
+     regions:
+       1: null
+     ```
+
+    Use the following configuration to serve the default DERP map (excluding New York) to nodes:
+
+    ```yaml title="config.yaml" hl_lines="6 7"
+    derp:
+      server:
+        enabled: false
+      urls:
+        - https://controlplane.tailscale.com/derpmap/default
+      paths:
+        - /etc/headscale/derp.yaml
+    ```
+
+=== "Provide custom DERP servers"
 
     The following sample `derp.yaml` references two custom regions (`custom-east` with ID 900 and `custom-west` with ID 901)
     with one custom DERP server in each region. Each DERP server offers DERP relay via HTTPS on tcp/443, support for captive
@@ -104,8 +126,8 @@ derp:
         - /etc/headscale/derp.yaml
     ```
 
-The embedded DERP server can also be enabled and is automatically added to the custom DERP map.
+Independent of the custom DERP map, you may choose to [enable the embedded DERP server and have it automatically added
-
+to the custom DERP map](#enable-embedded-derp).
 
 ### Verify clients