xclock: add page (#4224)

Co-authored-by: Starbeamrainbowlabs <sbrl@starbeamrainbowlabs.com>
Co-authored-by: Zlatan Vasović <zlatanvasovic@gmail.com>

pages/linux/xclock.md

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+# xclock
+
+> Display the time in analog or digital form.
+
+- Display an analog clock:
+
+`xclock`
+
+- Display a 24-hour digital clock with the hour and minute fields only:
+
+`xclock -digital -brief`
+
+- Display a digital clock using an strftime format string (see strftime(3)):
+
+`xclock -digital -strftime {{format}}`
+
+- Display a 24-hour digital clock with the hour, minute and second fields that updates every second:
+
+`xclock -digital -strftime '%H:%M:%S' -update 1`
+
+- Display a 12-hour digital clock with the hour and minute fields only:
+
+`xclock -digital -twelve -brief`