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

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

pages/common/pnmtosgi.md

@@ -0,0 +1,16 @@
+# pnmtosgi
+
+> Convert a PNM file to an SGI image file.
+> More information: <https://netpbm.sourceforge.net/doc/pnmtosgi.html>.
+
+- Convert a PNM image to an SGI image:
+
+`pnmtosgi {{path/to/input.pnm}} > {{path/to/output.sgi}}`
+
+- Specify whether or not compression should be used:
+
+`pnmtosgi -{{verbatim|rle}} {{path/to/input.pnm}} > {{path/to/output.sgi}}`
+
+- Write the specified string into the SGI image header's `imagename` field:
+
+`pnmtosgi -imagename {{string}} {{path/to/input.pnm}} > {{path/to/output.sgi}}`