fstopgm, lispmtopgm, pgmtofs, pgmtolispm, pgmtoppm, pnmalias: add pages (#12198)

pages/common/pgmtoppm.md

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+# pgmtoppm
+
+> Colorize a PGM image.
+> More information: <https://netpbm.sourceforge.net/doc/pgmtoppm.html>.
+
+- Map all greyscale values of the input image to all colors between the two specified colors:
+
+`pgmtoppm -black {{red}} --white {{blue}} {{path/to/input.pgm}} > {{path/to/output.ppm}}`
+
+- Map all greyscale values of the input image to colors according to the specified colormap:
+
+`pgmtoppm -map {{path/to/colormap.ppm}} {{path/to/input.pgm}} > {{path/to/output.ppm}}`