pamscale, pnmscale, pnmscalefixed: add pages (#11779)

pages/common/pnmscalefixed.md

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+# pnmscalefixed
+
+> Scale a PNM file quickly with possibly reduced quality.
+> See also: `pamscale`.
+> More information: <https://netpbm.sourceforge.net/doc/pnmscalefixed.html>.
+
+- Scale an image such that the result has the specified dimensions:
+
+`pnmscalefixed -width {{width}} -height {{height}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Scale an image such that the result has the specified width, keeping the aspect ratio:
+
+`pnmscalefixed -width {{width}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`
+
+- Scale an image such that its width and height is changed by the specified factors:
+
+`pnmscalefixed -xscale {{x_factor}} -yscale {{y_factor}} {{path/to/input.pnm}} > {{path/to/output.pnm}}`