chore(deploy): systemd unit, push.sh, one-time setup notes

deploy/void-server.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Void 2.0 server
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=void
+WorkingDirectory=/opt/void-server
+EnvironmentFile=/opt/void-server/.env
+ExecStart=/usr/bin/node /opt/void-server/server.js
+Restart=on-failure
+RestartSec=5
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=multi-user.target