pbmtox10bm, pbmtoxbm, ppmtoxpm, xbmtopbm, ximtoppm, xpmtoppm: add pages (#11407)

pages/common/ximtoppm.md

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+# ximtoppm
+
+> Convert a XIM file to a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ximtoppm.html>.
+
+- Convert an XIM image to a PPM image:
+
+`ximtoppm {{path/to/input_file.xim}} > {{path/to/output_file.ppm}}`
+
+- Store the transparency mask of the input image in the specified file:
+
+`ximtoppm --alphaout {{path/to/alpha_file.pbm}} {{path/to/input_file.xim}} > {{path/to/output_file.ppm}}`