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/pnmquant.md

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+# pnmquant
+
+> Quantize the colors in a PNM image into a smaller set.
+> This command is a combination of `pnmcolormap` and `pnmremap` and accepts the union of their options, except `-mapfile`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmquant.html>.
+
+- Generate an image using only `n_colors` or less colors as close as possible to the input image:
+
+`pnmquant {{n_colors}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`