ppmtoterm, ppmtoascii, pbmto4425: add pages; pbmtoascii: edit page (#12426)

pages/common/ppmtoascii.md

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+# ppmtoascii
+
+> Convert a PPM image to an ASCII image using ANSI terminal color codes.
+> See also: `ppmtoterm`, `pbmtoascii`, `pbmto4425`.
+> More information: <https://netpbm.sourceforge.net/doc/ppmtoascii.html>.
+
+- Convert a PPM image to an ASCII image, combining an area of 1x2 pixels into a character:
+
+`ppmtoascii {{path/to/input.ppm}} > {{path/to/output.txt}}`
+
+- Convert a PPM image to an ASCII image, combining an area of 2x4 pixels into a character:
+
+`ppmtoascii -2x4 {{path/to/input.ppm}} > {{path/to/output.txt}}`