systemd-ac-power: add page (#10196)

pages/linux/systemd-ac-power.md

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+# systemd-ac-power
+
+> Report whether the computer is connected to an external power source.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemd-ac-power.html>.
+
+- Silently check and return a 0 status code when running on AC power, and a non-zero code otherwise:
+
+`systemd-ac-power`
+
+- Additionally print `yes` or `no` to `stdout`:
+
+`systemd-ac-power --verbose`