Split off and fix RLlib demo notebook (#42)

The demo notebook used to contain everything: Customization, single-agent with SB3 and multi-agent with RLlib. It had several issues that are resolved in this PR:

* The SB3 version was pinned to some alpha that supported Gymnasium. Now, it's pinned to newly released version v2.0.0.
* RLlib was not pinned to any version, installed the latest one (2.5.1), which broke the notebook (#38 ). RLlib is now pinned to v2.5.1 and the notebook is adjusted to work correctly with it.
* SB3 and RLlib need different Gymnasium versions, leading to conflict within the notebook. This PR splits the RLlib part off into a separate notebook, avoiding the compatibility issues.

Closes #38 , #31, and #32

README.md

@@ -27,7 +27,10 @@ To maximize QoE globally, the policy must recognize that (1) the data rate of an
     <sup><a href="https://thenounproject.com/search/?q=base+station&i=1286474" target="_blank">Base station icon</a> by Clea Doltz from the Noun Project</sup>
 </p>
 
-**[Try mobile-env: ![Open in Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/stefanbschneider/mobile-env/blob/master/examples/demo.ipynb)**
+**Try mobile-env:**
+
+- Part I: Customizing mobile-env and single-agent RL with stable-baselines3: [![Open in Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/stefanbschneider/mobile-env/blob/master/examples/demo.ipynb)
+- Part II: Multi-agent RL on mobile-env with Ray RLlib: [![Open in Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/stefanbschneider/mobile-env/blob/master/examples/rllib.ipynb)
 
 Documentation and API: [ReadTheDocs](https://mobile-env.readthedocs.io/en/latest/)