docs: iio: Refactor index.rst

Refactor index.rst such that it contains a section for generic
documentation and a section for Kernel Drivers documentation.
Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Link: https://lore.kernel.org/r/20240221085848.991413-2-ramona.gradinariu@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/index.rst

@@ -9,6 +9,11 @@ Industrial I/O
 
    iio_configfs
 
-   ep93xx_adc
+Industrial I/O Kernel Drivers
+=============================
+
+.. toctree::
+   :maxdepth: 1
 
    bno055
+   ep93xx_adc