pbmtoascii: add page (#11289)

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/pbmtoascii.md

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+# pbmtoascii
+
+> Convert a PBM image to ASCII graphics.
+> More information: <https://netpbm.sourceforge.net/doc/pbmtoascii.html>.
+
+- Read a PBM file as input and produce an ASCII output:
+
+`pbmtoascii {{path/to/input_file.pbm}}`
+
+- Read a PBM file as input and save an ASCII output into a file:
+
+`pbmtoascii {{path/to/input_file.pbm}} > {{path/to/output_file}}`
+
+- Read a PBM file as input while setting the pixel mapping (defaults to 1x2):
+
+`pbmtoascii -{{1x2|2x4}} {{path/to/input_file.pbm}}`
+
+- Display version:
+
+`pbmtoascii -version`