Remove derp.yaml, add selfhosted example

This PR will promote fetching the derpmap directly from tailscale, so we
will remove our example, as it might easily get outdated.

Add a derp-example that shows how a user can also add their own derp
server.

derp-example.yaml

@@ -0,0 +1,18 @@
+# This file contains some of the official Tailscale DERP servers, 
+# shamelessly taken from https://github.com/tailscale/tailscale/blob/main/net/dnsfallback/dns-fallback-servers.json
+#
+# If you plan to somehow use headscale, please deploy your own DERP infra: https://tailscale.com/kb/1118/custom-derp-servers/
+regions: 
+  900:
+    regionid: 900
+    regioncode: custom
+    regionname: My Region
+    nodes:
+    - name: 1a
+      regionid: 1
+      hostname: myderp.mydomain.no
+      ipv4: 123.123.123.123
+      ipv6: "2604:a880:400:d1::828:b001"
+      stunport: 0
+      stunonly: false
+      derptestport: 0