pbmmake, pgmmake, ppmlabel, ppmmake, ppmmix, ppmntsc, ppmpat: add pages (#11435)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/common/pgmmake.md

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+# pgmmake
+
+> Create PGM image with a uniform gray level.
+> More information: <https://netpbm.sourceforge.net/doc/pgmmake.html>.
+
+- Create PGM image with a uniform gray level (specified as a number between 0 and 1) and the specified dimensions:
+
+`pgmmake {{graylevel}} {{width}} {{height}} > {{path/to/output_file.pgm}}`