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

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

pages/common/ppmmix.md

@@ -0,0 +1,8 @@
+# ppmmix
+
+> Blend together two PPM images.
+> More information: <https://netpbm.sourceforge.net/doc/ppmmix.html>.
+
+- Blend the specified PPM images using fadefactor to control the weight of each image:
+
+`ppmmix {{fadefactor}} {{path/to/input_file1.ppm}} {{path/to/input_file2.ppm}} > {{path/to/output_file.ppm}}`