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/pnmmargin.md

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+# pnmmargin
+
+> Add a border to a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmmargin.html>.
+
+- Add a border of the specified size to a PNM image:
+
+`pnmmargin {{size}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Specify the color of the border:
+
+`pnmmargin -color {{color}} {{size}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`