DocBook: Add initial documentation for IIO

This is intended to help developers faster find their way
inside the Industrial I/O core and reduce time spent on IIO
drivers development.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Acked-by: Crt Mori <cmo@melexis.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>