docs(ex3EKF): ddd reference to Original Ex3EKF env description

This commit updates the documentation to include the original source
where the Ex3EKF environment was first described. This addition provides
readers with the context and foundational literature necessary to
understand the development and application of the Ex3EKF environment
more comprehensively.

stable_gym/envs/classic_control/ex3_ekf/README.md

@@ -1,6 +1,6 @@
 # Ex3EKF gymnasium environment
 
-A gymnasium environment for a noisy master-slave system. This environment can be used to train an RL-based stationary Kalman filter.
+A gymnasium environment for a noisy master-slave system. This environment can be used to train an RL-based stationary Kalman filter. First presented by [Wu et al. 2023](https://www.sciencedirect.com/science/article/pii/S0005109823001528).
 
 ## Observation space
 

stable_gym/envs/classic_control/ex3_ekf/ex3_ekf.py

@@ -15,7 +15,7 @@ class Ex3EKF(gym.Env):
     Description:
         The goal of the agent in the Ex3EKF environment is to act in such a way that
         estimator perfectly estimated the original noisy system. By doing this it serves
-        as a RL based stationary Kalman filter.
+        as a RL based stationary Kalman filter. First presented by `Wu et al. 2023`_.
 
     Observation:
         **Type**: Box(4)
@@ -64,6 +64,8 @@ class Ex3EKF(gym.Env):
         t (float): The current time step.
         dt (float): The environment step size. Also available as :attr:`.tau`.
         sigma (float): The variance of the system noise.
+
+    .. _`Wu et al. 2023`: https://www.sciencedirect.com/science/article/pii/S0005109823001528
     """  # noqa: E501
 
     def __init__(