meta(docs): Add systemd example files

examples/update-firewall-source.service.example

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+[Unit]
+Description=firewalld direct rules generator
+After=multi-user.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=no
+Environment='PATH=/usr/local/sbin:/usr/local/bin:/usr/bin' 'UFS_LOGLEVEL=INFO'
+ExecStart=/opt/miniconda3/envs/update-firewall-source/bin/python /opt/python/update-firewall-source/dev/update-firewall-source.py
+
+[Install]
+WantedBy=multi-user.target

examples/update-firewall-source.timer.example

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+[Unit]
+Description=Run firewalld direct rules generator
+
+[Timer]
+OnCalendar=
+OnCalendar=4,5,6:00,15,30,45 Asia/Shanghai
+OnCalendar=1,10,14,18,22:00 Asia/Shanghai
+OnBootSec=5min
+RandomizedDelaySec=2min
+Persistent=true
+
+[Install]
+WantedBy=timers.target