pamfunc, ppmchange, ppmcie, ppmcolormask, ppmdim, ppmdist, ppmfade, ppmforge: add pages (#11449)

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/ppmdist.md

@@ -0,0 +1,12 @@
+# ppmdist
+
+> Produce a grayscale version of a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ppmdist.html>.
+
+- Produce a grayscale version of the specified PPM image:
+
+`ppmdist {{path/to/input.ppm}} > {{path/to/output.pgm}}`
+
+- Specify the method used to map colors to graylevels:
+
+`ppmdist -{{frequency|intensity}} {{path/to/input.ppm}} > {{path/to/output.pgm}}`