Apply RTF fixups which improves our .rtf generation:

• Tell the location where Jade/OpenJade will look for the PNG images.
• Use rtf-nopng as extension name for Jade/OpenJade generated RTF files.
• Use fixrtf(1) from textproc/fixrtf to make necessary fixups on generated RTFs, embedding PNGs into the intermediate RTF and save them into the new *.rtf files.
• Retire rtf.tar target which is no longer necessary.

Please note that you have to upgrade the docproj toolset or
this commit would break the build.

PR: docs/93965
Submitted by: intron <intron intron ac>
Approved by: doceng