ModuleDocumentation: add autotune categorie

Tools/px4moduledoc/srcparser.py

@@ -14,7 +14,7 @@ class ModuleDocumentation(object):
     # If you add categories or subcategories, they also need to be added to the
     # TOC in https://github.com/PX4/Devguide/blob/master/en/SUMMARY.md
     valid_categories = ['driver', 'estimator', 'controller', 'system',
-                        'communication', 'command', 'template', 'simulation']
+                        'communication', 'command', 'template', 'simulation', 'autotune']
     valid_subcategories = ['', 'distance_sensor', 'imu', 'airspeed_sensor',
                            'magnetometer', 'baro', 'optical_flow']