samples: doc: Added README for RTIO sample

Added README.rst for sensor_batch_processing sample.
Added rtio.rst listing samples from RTIO subsystem.

Signed-off-by: Michal Piekos <michal.piekos@wp.pl>

samples/subsys/rtio/rtio.rst

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+.. zephyr:code-sample-category:: rtio
+   :name: Real Time I/O (RTIO)
+   :show-listing:
+
+   Samples that demonstrate the :ref:`rtio` subsystem.

samples/subsys/rtio/sensor_batch_processing/README.rst

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+.. zephyr:code-sample:: sensor_batch_processing
+   :name: Sensor batch processing
+   :relevant-api: rtio
+
+   Implement a sensor device using RTIO for asynchronous data processing.
+
+Overview
+********
+
+This sample application demonstrates the use of the :ref:`rtio` framework for
+doing asynchronous operation chains.
+Application uses :ref:`rtio` with mempool API to fetch data from virtual sensor
+and displays it on the console.
+
+Requirements
+************
+
+* A board with flash support or native_sim target
+
+Building and Running
+********************
+
+This sample can be found under :zephyr_file:`samples/subsys/rtio` in the Zephyr tree.
+
+The sample can be built for most platforms, the following commands build the
+application for the native_sim target.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/subsys/rtio
+   :board: native_sim
+   :goals: build run
+   :compact:
+
+When running, the output on the console shows the operations of
+submitting for, consuming, and processing the sensor data.
+
+Sample Output
+=============
+
+.. code-block:: console
+
+   *** Booting Zephyr OS build v4.0.0-1260-gbaa49f6f32d5 ***
+   I: Submitting 4 read requests
+   D: sensor@0: buf_len = 16, buf = 0x8056430
+   D: sensor@0: buf_len = 16, buf = 0x8056440
+   D: sensor@0: buf_len = 16, buf = 0x8056450
+   D: sensor@0: buf_len = 16, buf = 0x8056460
+   D: Consumed completion event 0
+   D: Consumed completion event 1
+   D: Consumed completion event 2
+   D: Consumed completion event 3
+   I: Start processing 4 samples
+   D: Sample data:
+   D: 00 01 02 03 04 05 06 07 |........
+   D: 08 09 0a 0b 0c 0d 0e 0f |........
+   D: Sample data:
+   D: 10 11 12 13 14 15 16 17 |........
+   D: 18 19 1a 1b 1c 1d 1e 1f |........
+   D: Sample data:
+   D: 20 21 22 23 24 25 26 27 | !"#$%&'
+   D: 28 29 2a 2b 2c 2d 2e 2f |()*+,-./
+   D: Sample data:
+   D: 30 31 32 33 34 35 36 37 |01234567
+   D: 38 39 3a 3b 3c 3d 3e 3f |89:;<=>?
+   D: sensor@0: buf_len = 16, buf = 0x8056470
+   D: sensor@0: buf_len = 16, buf = 0x8056480
+   D: sensor@0: buf_len = 16, buf = 0x8056490
+   I: Finished processing 4 samples
+   I: Submitting 4 read requests
+   D: sensor@0: buf_len = 16, buf = 0x8056430
+   D: sensor@0: buf_len = 16, buf = 0x8056440
+   D: sensor@0: buf_len = 16, buf = 0x8056450
+   D: sensor@0: buf_len = 16, buf = 0x8056460
+   D: Consumed completion event 0
+   ...