systemctl-poweroff: add page (#18292)

Co-authored-by: Managor <42655600+Managor@users.noreply.github.com>

pages/linux/systemctl-poweroff.md

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+# systemctl-poweroff
+
+> Power off the system.
+> See also: `poweroff`.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemctl.html#poweroff>.
+
+- Power off the system:
+
+`systemctl poweroff`
+
+- Power off the system immediately without asking services to stop gracefully:
+
+`systemctl poweroff --force`
+
+- Power off the system immediately without sending notifications to logged-in users:
+
+`systemctl poweroff --force --no-wall`