pnmsmooth: add page (#11757)

pages/common/pnmsmooth.md

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+# pnmsmooth
+
+> Smooth out a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmsmooth.html>.
+
+- Smooth out a PNM image using a convolution matrix of size 3x3:
+
+`pnmsmooth {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Smooth out a PNM image using a convolution matrix of size width times height:
+
+`pnmsmooth -width {{width}} -height {{height}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`