docs: iio: add documentation for adxl380 driver

Add documentation for adxl380 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>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://patch.msgid.link/20240708104114.29894-3-antoniu.miclaus@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/index.rst

@@ -21,5 +21,6 @@ Industrial I/O Kernel Drivers
    ad7944
    adis16475
    adis16480
+   adxl380
    bno055
    ep93xx_adc