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

* pamdepth, pamdither, pamditherbw: add pages

* pnmdepth: add alias page

pages/common/ppmdither.md

@@ -0,0 +1,16 @@
+# 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}}`