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

* pamdepth, pamdither, pamditherbw: add pages

* pnmdepth: add alias page

pages/common/pamditherbw.md

@@ -0,0 +1,20 @@
+# 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}}`