add metrics docs

Author: LarsKue

bayesflow/metrics/__init__.py

@@ -1,3 +1,7 @@
+"""
+A collection of `keras.Metric <https://keras.io/api/metrics/base_metric/#metric-class>`__\ s for evaluating the performance of models.
+"""
+
 from . import functional
 from .maximum_mean_discrepancy import MaximumMeanDiscrepancy
 from .root_mean_squard_error import RootMeanSquaredError