cal: improve util-linux page and split POSIX, osx and *BSD (#12172)

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/openbsd/cal.md

@@ -0,0 +1,32 @@
+# cal
+
+> Display a calendar with the current day highlighted.
+> More information: <https://man.openbsd.org/cal>.
+
+- Display a calendar for the current month:
+
+`cal`
+
+- Display a calendar for a specific year:
+
+`cal {{year}}`
+
+- Display a calendar for a specific month and year:
+
+`cal {{month}} {{year}}`
+
+- Display a calendar for the current [y]ear:
+
+`cal -y`
+
+- Display [j]ulian days (starting from one, numbered from January 1):
+
+`cal -j`
+
+- Use [m]onday as week start instead of Sunday:
+
+`cal -m`
+
+- Number [w]eek numbers (incompatible with `-j`):
+
+`cal -w`