fstopgm, lispmtopgm, pgmtofs, pgmtolispm, pgmtoppm, pnmalias: add pages (#12198)

pages/common/pnmalias.md

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+# pnmalias
+
+> Apply antialiasing onto a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/pnmalias.html>.
+
+- Perform antialiasing on a PNM image, taking black pixels as background and white pixels as foreground:
+
+`pnmalias {{path/to/input.pnm}} > {{path/to/output.ppm}}`
+
+- Explicitly specify the background and foreground color:
+
+`pnmalias -bcolor {{background_color}} -fcolor {{foreground_color}} {{path/to/input.pnm}} > {{path/to/output.ppm}}`
+
+- Apply altialiasing to foreground pixels only:
+
+`pnmalias -fonly {{path/to/input.pnm}} > {{path/to/output.ppm}}`
+
+- Apply antialiasing to all surrounding pixels of background pixels:
+
+`pnmalias -balias {{path/to/input.pnm}} > {{path/to/output.ppm}}`