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/pnmnlfilt.md

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+# pnmnlfilt
+
+> Apply a non-linear filter onto a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmnlfilt.html>.
+
+- Apply the "alpha trimmed mean" filter with the specified alpha and radius values onto the PNM image:
+
+`pnmnlfilt {{0.0..0.5}} {{radius}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Apply the "optimal estimation smoothing" filter with the specified noise threshold and radius onto the PNM image:
+
+`pnmnlfilt {{1.0..2.0}} {{radius}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`
+
+- Apply the "edge enhancement" filter with the specified alpha and radius onto the PNM image:
+
+`pnmnlfilt {{-0.9..(-0.1)}} {{radius}} {{path/to/image.pnm}} > {{path/to/output.pnm}}`