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/gemtopbm.md

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+# gemtopbm
+
+> This command is superseded by `gemtopnm`.
+> More information: <https://netpbm.sourceforge.net/doc/gemtopbm.html>.
+
+- View documentation for the current command:
+
+`tldr gemtopnm`

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`

pages/common/pamtotga.md

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+# pamtotga
+
+> Convert a Netpbm image to a TrueVision Targa file.
+> More information: <https://netpbm.sourceforge.net/doc/pamtotga.html>.
+
+- Convert a Netpbm image to a TrueVision Targa file:
+
+`pamtotga {{path/to/file.pam}} > {{path/to/output.tga}}`
+
+- Specify the color map of the output image:
+
+`pamtotga -{{cmap|cmap16|mono|rgb}} {{path/to/file.pam}} > {{path/to/output.tga}}`
+
+- Display version:
+
+`pamtotga -version`

pages/common/ppmtotga.md

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+# ppmtotga
+
+> This command is superseded by `pamtotga`.
+> More information: <https://netpbm.sourceforge.net/doc/ppmtotga.html>.
+
+- View documentation for the current command:
+
+`tldr pamtotga`

pages/common/tgatoppm.md

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+# tgatoppm
+
+> Convert a TrueVision Targa file to a Netpbm image.
+> More information: <https://netpbm.sourceforge.net/doc/tgatoppm.html>.
+
+- Convert a TrueVision Targa file to a PPM image:
+
+`tgatoppm {{path/to/file.tga}} > {{path/to/output.ppm}}`
+
+- Dump information from the TGA header to `stdout`:
+
+`tgatoppm --headerdump {{path/to/file.tga}} > {{path/to/output.ppm}}`
+
+- Write the transparency channel values of the input image to the specified file:
+
+`tgatoppm --alphaout {{path/to/transparency_file.pgm}} {{path/to/file.tga}} > {{path/to/output.ppm}}`
+
+- Display version:
+
+`tgatoppm -version`