anytopnm: add page (#10908)

pages/common/anytopnm.md

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+# anytopnm
+
+> Converts an arbitrary type of image file to common image formats.
+> More information: <https://netpbm.sourceforge.net/doc/anytopnm.html>.
+
+- Convert an input image to PBM, PGM, or PPM format irrespective of the input type:
+
+`anytopnm {{path/to/input}} > {{path/to/output.pnm}}`
+
+- Display version:
+
+`anytopnm -version`