pnmmargin, pnmmontage, pnmnlfilt, pnmnorm, pnmpad, pnmpaste: add pages (#12187)

* pnmmargin, pnmmontage, pnmnlfilt, pnmnorm, pnmpad, pnmpaste: add pages

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Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

pages/common/pnmmontage.md

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+# pnmmontage
+
+> Create a montage from multiple PNM images.
+> More information: <https://netpbm.sourceforge.net/doc/pnmmontage.html>.
+
+- Produce a packing of the specified images:
+
+`pnmmontage {{path/to/image1.pnm path/to/image2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Specify the quality of the packing (Note: larger values produce smaller packings but take longer to compute.):
+
+`pnmmontage -{{0..9}} {{path/to/image1.pnm path/to/image2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Produce a packing that is not larger than `p` percent of the optimal packing:
+
+`pnmmontage -quality {{p}} {{path/to/image1.pnm path/to/image2.pnm ...}} > {{path/to/output.pnm}}`
+
+- Write the positions of the input files within the packed image to a machine-readable file:
+
+`pnmmontage -data {{path/to/datafile}} {{path/to/image1.pnm path/to/image2.pnm ...}} > {{path/to/output.pnm}}`