hipstopgm: add page (#11197)

pages/common/hipstopgm.md

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+# hipstopgm
+
+> Read a HIPS file as input and return a PGM image as output.
+> If the HIPS file contains more than one frame in sequence, `hipstopgm` will concatenate all the frames vertically.
+> More information: <https://netpbm.sourceforge.net/doc/hipstopgm.html>.
+
+- Convert a HIPS file into a PGM image:
+
+`hipstopgm {{path/to/file.hips}}`
+
+- Suppress all informational messages:
+
+`hipstopgm -quiet`
+
+- Display version:
+
+`hipstopgm -version`