ARC: doc overhaul for nSIM board (with few fixes to MWDT docs)

Completely rewrite nSIM board documentation with additional fixes
to ARC MWDT toolchain documentation.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>