dc: add page (#1807)

pages/common/dc.md

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+# dc
+
+> An arbitrary precision calculator. Uses reverse polish notation (RPN).
+
+- Run calculator in interactive mode:
+
+`dc`
+
+- Execute dc script in file:
+
+`dc -f {{file}}`
+
+- Calculate 4 times 5 [4 5 *], subtract 17 [17 -], and [p]rint the output (using echo):
+
+`echo "4 5 * 17 - p"| dc`
+
+- Set number of decimal places to 7 [7 k], calculate 5 divided by -3 [5 _3 /] and [p]rint (using dc -e):
+
+`dc -e "7 k 5 _3 / p"`
+
+- Calculate the golden ratio, phi: Set number of decimal places to 100 [100 k], square root of 5 [5 v] plus 1 [1 +], divided by 2 [2 /], and [p]rint result:
+
+`dc -e "100 k 5 v 1 + 2 / p"`