ppmflash: add page (#11116)

* ppmflash: add page

* Update pages/common/ppmflash.md

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>

* Update ppmflash.md

---------

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>

pages/common/ppmflash.md

@@ -0,0 +1,12 @@
+# ppmflash
+
+> Brighten a PPM image file.
+> More information: <https://netpbm.sourceforge.net/doc/ppmflash.html>.
+
+- Generate a PPM image as output that is `flashfactor` times brighter than the input PPM image:
+
+`ppmflash {{flashfactor}} {{path/to/file.ppm}} > {{path/to/file.ppm}}`
+
+- Display version:
+
+`ppmflash -version`