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}}`