added sample plot into docs

mlair/plotting/data_insight_plotting.py

@@ -456,7 +456,17 @@ class PlotPeriodogram(AbstractPlotClass):  # pragma: no cover
 
     If data consists on different sampling rates, a separate plot is create for each sampling.
 
-    To work properly, the data handler must have the attribute `.id_class._data`.
+    .. image:: ../../../../../_source/_plots/periodogram.png
+        :width: 400
+
+    .. note::
+        This plot is not included in the default plot list. To use this plot, add "PlotPeriodogram" to the `plot_list`.
+
+    .. warning::
+        This plot is highly sensitive to the data handler structure. Therefore, it is highly likely that this method is
+        not compatible with any custom data handler. Proven data handlers are `DefaultDataHandler`,
+        `DataHandlerMixedSampling`, `DataHandlerMixedSamplingWithFilter`. To work properly, the data handler must have
+        the attribute `.id_class._data`.
     """
 
     def __init__(self, generator: Dict[str, DataCollection], plot_folder: str = ".", plot_name="periodogram",