docs: qemu/virt platform documentation fixes and updates

- Correct the payload address in the RV64 examples that use
  fw_jump.elf.

- Change the qemu console configuration in the examples from
  "-display none -serial stdio" to "-nographic". This results in
  qemu handing down a CTRL-C on the emulated console to the VM
  instead of terminating the qemu process.

- Provide examples for RV32.

- Various text corrections.

Signed-off-by: Karsten Merker <merker@debian.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>