doc: board: qemu-mips.rst: fix code examples

Code sections should be syntax highlighted as bash.
Comment lines in code should start with a hash sign ('#') but code lines
should not.
Most commands can be executed as normal users. Prepend those commands
requiring elevated authorization with 'sudo'.
dd does not have a parameter cout.
sfdisk does not have a -C parameter on Debian Buster.
Provide the necessary input to sfdisk.
Creating a partition of length zero makes no sense.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>