pnmarith, pamarith: add pages (#12422)

* pnmarith, pamarith: add pages

---------

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

pages/common/pamarith.md

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+# pamarith
+
+> Apply a binary function on two Netpbm images.
+> See also: `pamfunc`.
+> More information: <https://netpbm.sourceforge.net/doc/pamarith.html>.
+
+- Apply the specified binary function pixel-wise on the two specified images (which must be of the same size):
+
+`pamarith -{{add|subtract|multiply|divide|difference|minimum|maximum|...}} {{path/to/image1.pam|pbm|pgm|ppm}} {{path/to/image2.pam|pbm|pgm|ppm}}`

pages/common/pnmarith.md

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+# pnmarith
+
+> This command is superseded by `pamarith`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmarith.html>.
+
+- View documentation for the current command:
+
+`tldr pamarith`