Base: Add shutdown(1) manpage

Author: https://github.com/supercomputer7
Commit: https://github.com/SerenityOS/serenity/commit/a875083739c
Pull-request: https://github.com/SerenityOS/serenity/pull/11314

Base/usr/share/man/man1/shutdown.md

@@ -0,0 +1,18 @@
+## Name
+
+shutdown - Power off the machine
+
+## Synopsis
+
+```**sh
+$ shutdown
+```
+
+## Description
+
+`shutdown` instructs the kernel to power off the machine immediately.
+
+## Notes
+
+The `shutdown` utility opens the `/sys/firmware/power_state` node and writes the magic value "2"
+to instruct the kernel to power off the machine.