pamdepth, ppmdither, pamditherbw, pnmdepth: add pages (#11379)

* pamdepth, pamdither, pamditherbw: add pages

* pnmdepth: add alias page

pages/common/pamdepth.md

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+# pamdepth
+
+> Reduce the depth (i.e. color resolution) in an image.
+> More information: <https://netpbm.sourceforge.net/doc/pamdepth.html>.
+
+- Read a PBM image, set its maxval and save it to a file:
+
+`pamdepth {{maxval}} {{path/to/image.pbm}} > {{path/to/file.pbm}}`

pages/common/pamditherbw.md

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+# pamditherbw
+
+> Apply dithering to a greyscale image, i.e. turn it into a pattern of black and white pixels that look the same as the original greyscale.
+> More information: <https://netpbm.sourceforge.net/doc/pamditherbw.html>.
+
+- Read a PGM image, apply dithering and save it to a file:
+
+`ppmditherbw {{path/to/image.pgm}} > {{path/to/file.pgm}}`
+
+- Use the specified quantization method:
+
+`ppmditherbw -{{floyd|fs|atkinson|threshold|hilbert|...}} {{path/to/image.pgm}} > {{path/to/file.pgm}}`
+
+- Use the atkinson quantization method and the specified seed for a pseudo-random number generator:
+
+`ppmditherbw -atkinson -randomseed {{1337}} {{path/to/image.pgm}} > {{path/to/file.pgm}}`
+
+- Specify the thresholding value for quantization methods that perform some sort of thresholding:
+
+`ppmditherbw -{{fs|atkinson|thresholding}} -value {{0.3}} {{path/to/image.pgm}} > {{path/to/file.pgm}}`

pages/common/pnmdepth.md

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+# pnmdepth
+
+> This command is an alias of `pamdepth`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmdepth.html>.
+
+- View documentation for the original command:
+
+`tldr pamdepth`

pages/common/ppmdither.md

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+# ppmdither
+
+> Reduce the number of colors in an image by applying dithering.
+> More information: <https://netpbm.sourceforge.net/doc/ppmdither.html>.
+
+- Read a PPM image, apply dithering and save it to a file:
+
+`ppmdither {{path/to/image.ppm}} > {{path/to/file.ppm}}`
+
+- Specify the desired number of shades for each primary color:
+
+`ppmdither -red {{2}} -green {{3}} -blue {{2}} {{path/to/image.ppm}} > {{path/to/file.ppm}}`
+
+- Specify the dimensions of the dithering matrix:
+
+`ppmdither -dim {{2}} {{path/to/image.ppm}} > {{path/to/file.ppm}}`