config: set marker_listen_port default to 3070 and document in config templates

Change the marker UDP sink default from port=0 (OS-chosen) to 3070 for
predictable firewall/debugging/monitoring.

Add marker_listen_host and marker_listen_port with documentation to
config_samples/gns3_server.conf and scripts/gns3_server.conf.

scripts/gns3_server.conf

@@ -4,3 +4,7 @@ compute_password = gns3
 skills_repo_url = https://github.com/yueguobin/GNS3-Skills.git
 skills_repo_branch = main
 skills_auto_update = false
+
+; Marker (traffic-insight) UDP sink port for uBridge MARK signals
+; Set to 0 for OS-chosen port
+marker_listen_port = 3070