pnmtorast, pnmtorle, pnmtosgi, pnmtosir, rasttopnm, rletopnm, sgitopnm, sirtopnm: add pages (#11410)

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

pages/common/rasttopnm.md

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+# rasttopnm
+
+> Convert a Sun rasterfile to a PNM file.
+> More information: <https://netpbm.sourceforge.net/doc/rasttopnm.html>.
+
+- Convert a RAST image to a PNM file:
+
+`rasttopnm {{path/to/input.rast}} > {{path/to/output.pnm}}`
+
+- Use the color map indices in the raster if they are color values:
+
+`rasttopnm -index {{path/to/input.rast}} > {{path/to/output.pnm}}`