gemtopbm, gemtopnm, pamtotga, ppmtotga, tgatoppm: add pages (#11402)

* gemtopbm, gemtopnm, pamtotga, ppmtotga, tgatoppm: add pages

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Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/common/gemtopnm.md

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+# gemtopbm
+
+> Convert a GEM image file into a PNM image.
+> More information: <https://netpbm.sourceforge.net/doc/gemtopnm.html>.
+
+- Convert a GEM image file to a PNM image:
+
+`gemtopbm {{path/to/file.img}} > {{path/to/output.pbm}}`
+
+- Describe the contents of the specified GEM image:
+
+`gemtopbm -d {{path/to/file.img}}`
+
+- Display version:
+
+`gemtopbm -version`