Add MarsExplorer-Gymnasium environment (#1535)

Author: markub3327

docs/environments/third_party_environments.md

@@ -307,6 +307,15 @@ goal-RL ([Gymnasium-Robotics](https://robotics.farama.org/)).
 
   ![Gymnasium version dependency](https://img.shields.io/badge/Gymnasium-v1.0.0-blue)
   ![GitHub stars](https://img.shields.io/github/stars/dfki-ric/deformable_gym)
+
+- [MarsExplorer-Gymnasium: Gymnasium-compatible Mars rover exploration environment](https://github.com/markub3327/MarsExplorer-Gymnasium)
+
+  ![Gymnasium version dependency](https://img.shields.io/badge/Gymnasium-v1.2.3-blue)
+  ![GitHub stars](https://img.shields.io/github/stars/markub3327/MarsExplorer-Gymnasium)
+
+  MarsExplorer-Gymnasium is a fully Gymnasium-compatible environment for autonomous Mars rover exploration. It enables research on terrain coverage, navigation, and exploration efficiency in unknown, procedurally generated landscapes, supporting modern deep reinforcement learning algorithms through the standard Gymnasium API.
+
+
 ### Telecommunication Systems environments
 *Interact and/or manage wireless and/or wired telecommunication systems.*
 - [mobile-env: Environments for coordination of wireless mobile networks](https://github.com/stefanbschneider/mobile-env)