docs: iio: add documentation for adis16475 driver

Add documentation for adis16475 driver which describes
the driver device files and shows how the user may use the
ABI for various scenarios (configuration, measurement, etc.).

Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Link: https://lore.kernel.org/r/20240221085848.991413-4-ramona.gradinariu@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>