Documentation/zio: removed (we have doc/ already)

Some of the information in there was old, and we should rather extract
the files in Documentaion/zio/ from the doc/zio-manual.txt .
Currently, just remove the old information.

Please, users, read material in doc/ instead (or the pdf on ohwr.org).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>