docs: rewrite `part-bench` doc

[SebastienMorais]

Seems like part-bench's doc is a straight copy paste from mesh-part.

[hhirtz]

Yes, they share the same man page because usage is very similar. :) The options specific to one of both tools are at the bottom of the OPTIONS section.

FYI, github renders .scd files as markdown files but it's incorrect. To view these files in a proper way I suggest use bash and scdoc:

apt install scdoc
man <(scdoc <tools/doc/part-bench.1.scd)

[hhirtz]

Alternatively, you can read them in a web browser with

scdoc <tools/doc/part-bench.1.scd | groff -mandoc -Thtml >/tmp/part-bench.html
firefox /tmp/part-bench.html