pamrgbatopng, pamtopng, pngtopam: add pages (#12399)

pages/common/pngtopam.md

@@ -0,0 +1,21 @@
+# pngtopam
+
+> Convert a PNG image to a Netpbm image.
+> See also: `pamtopng`.
+> More information: <https://netpbm.sourceforge.net/doc/pngtopam.html>.
+
+- Convert the specified PNG image to a Netpbm image:
+
+`pngtopam {{path/to/image.png}} > {{path/to/output.pam}}`
+
+- Create an output image that includes both the main image and transparency mask of the input image:
+
+`pngtopam -alphapam {{path/to/image.png}} > {{path/to/output.pam}}`
+
+- Replace transparent pixels by the specified color:
+
+`pngtopam -mix -background {{color}} {{path/to/image.png}} > {{path/to/output.pam}}`
+
+- Write tEXt chunks found in the input image to the specified text file:
+
+`pngtopam -text {{path/to/file.txt}} {{path/to/image.png}} > {{path/to/output.pam}}`