pi3topbm, pbmtopi3: add pages (#12527)

* pi3toppm, ppmtopi3: add pages

---------

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

pages/common/pbmtopi3.md

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+# pbmtopi3
+
+> Convert a PBM image to an Atari Degas PI3 image.
+> See also: `pi3topbm`.
+> More information: <https://netpbm.sourceforge.net/doc/pbmtopi3.html>.
+
+- Convert a PBM image to an Atari Degas PI3 image:
+
+`pbmtopi3 {{path/to/image.pbm}} > {{path/to/atari_image.pi3}}`

pages/common/pi3topbm.md

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+# pi3topbm
+
+> Convert an Atari Degas PI3 image to PBM image.
+> See also: `pbmtopi3`.
+> More information: <https://netpbm.sourceforge.net/doc/pi3topbm.html>.
+
+- Convert an Atari Degas PI3 image to PBM image:
+
+`pi1topbm {{path/to/atari_image.pi3}} > {{path/to/output_image.pbm}}`