Add systemd example service to docs
ADD
docs/systemd/monolith-example.service
+[Unit]
+Description=Dendrite (Matrix Homeserver)
+After=syslog.target
+After=network.target
+After=postgresql.service
+
+[Service]
+RestartSec=2s
+Type=simple
+User=dendrite
+Group=dendrite
+WorkingDirectory=/opt/dendrite/
+ExecStart=/opt/dendrite/bin/dendrite-monolith-server
+Restart=always
+
+[Install]
+WantedBy=multi-user.target

Add systemd example service to docs (#1512)

Signed-off-by: Pieter Hollander <git@pieterhollander.nl>