pnmpsnr: add page (#12180)

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

pages/common/pnmpsnr.md

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+# pnmpsnr
+
+> Compute the difference between two images.
+> More information: <https://netpbm.sourceforge.net/doc/pnmpsnr.html>.
+
+- Compute the difference, i.e. the peak signal-to-noise ratio (PSNR) between two images:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}}`
+
+- Compare the color components rather than the luminance and chrominance components of the images:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -rgb`
+
+- Run in comparison mode, i.e. only output `nomatch` or `match` depending on whether the computing PSNR exceeds `n` or not:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -target {{n}}`
+
+- Run in comparison mode and compare the individual image components, i.e. Y, Cb, and Cr, to the corresponding thresholds:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -target1 {{threshold_Y}} -target2 {{threshold_Cb}} -target3 {{threshold_Cr}}`
+
+- Run in comparison mode and compare the individual image components, i.e. red, green, and blue to the corresponding thresholds:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -rgb -target1 {{threshold_red}} -target2 {{threshold_green}} -target3 {{threshold_blue}}`
+
+- Produce machine-readable output:
+
+`pnmpsnr {{path/to/file1.pnm}} {{path/to/file2.pnm}} -machine`