Updated ICON doc with information on usage of extract_levels preprocessor (#1903)

Co-authored-by: Axel Lauer <axel.lauer@dlr.de>

Author: schlunma

doc/quickstart/find_data.rst

@@ -363,6 +363,23 @@ Key           Description                   Default value if not specified
               file                          default DRS is used)
 ============= ============================= =================================
 
+.. hint::
+
+   To use the :func:`~esmvalcore.preprocessor.extract_levels` preprocessor on
+   native ICON data, you need to specify the name of the vertical coordinate
+   (e.g., ``coordinate: air_pressure``) since native ICON output usually
+   provides a 3D air pressure field instead of a simple 1D vertical coordinate.
+   Example:
+
+   .. code-block:: yaml
+
+    preprocessors:
+      extract_500hPa_level_from_icon:
+        extract_levels:
+          levels: 50000
+          scheme: linear
+          coordinate: air_pressure
+
 .. hint::
 
    In order to read cell area files (``areacella`` and ``areacello``), one