pi: add page (#4653)

pages/linux/pi.md

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+# pi
+
+> Compute decimal Archimedes' constant Pi on the command line.
+> More information: <http://manpages.ubuntu.com/manpages/trusty/man1/pi.1.html>.
+
+- Display 100 decimal digits of Archimedes' constant Pi:
+
+`pi`
+
+- Display a specified number of decimal digits of Archimedes' constant Pi:
+
+`pi {{number}}`
+
+- Display help:
+
+`pi --help`
+
+- Display version:
+
+`pi --version`
+
+- Display recommended readings:
+
+`pi --bibliography`