[signal,proxy,relay] Add service configuration examples

Operators need concrete schemas for the new config-file flags.

Add validated YAML examples for Relay/STUN, Signal, and Proxy.

relay/config.example.yaml

@@ -0,0 +1,22 @@
+# Command-line flags override environment variables, which override this file.
+listenAddress: ":443"
+exposedAddress: "rels://relay.example.com:443"
+metricsPort: 9090
+
+# Use either static certificates or Let's Encrypt.
+letsencryptEmail: ""
+letsencryptDataDir: ""
+letsencryptDomains: []
+letsencryptAWSRoute53: false
+tlsCertFile: "/etc/netbird/tls.crt"
+tlsKeyFile: "/etc/netbird/tls.key"
+
+authSecret: "replace-with-a-strong-secret"
+logLevel: "info"
+logFile: "console"
+healthcheckListenAddress: ":9000"
+trustedProxies: ""
+
+enableSTUN: true
+stunPorts: [3478]
+stunLogLevel: "info"