pamflip, pnmcolormap, pnmquant, pnmremap, pnmrotate, ppmquant: add pages (#11800)

* pamflip, pnmcolormap, pnmquant, pnmremap, pnmrotate, ppmquant: add pages

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Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/pnmcolormap.md

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+# pnmcolormap
+
+> Create quantization color map for a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmcolormap.html>.
+
+- Generate an image using only `n_colors` or less colors as close as possible to the input image:
+
+`pnmcolormap {{n_colors}} {{path/to/input.pnm}} > {{path/to/output.ppm}}`
+
+- Use the splitspread strategy for determining the output colors, possibly producing a better result for images with small details:
+
+`pnmcolormap -splitspread {{n_colors}} {{path/to/input.pnm}} > {{path/to/output.ppm}}`
+
+- Sort the resulting colormap, which is useful for comparing colormaps:
+
+`pnmcolormap -sort {{path/to/input.pnm}} > {{path/to/output.ppm}}`