Add CartPole Tutorial Notebook to Demonstrate Custom Task Creation

[ttktjmt]

This PR adds a new Jupyter notebook to the notebooks/ directory for demonstrating customization usage of mjlab on Google Colab. The notebook shows how to create a custom RL task with mjlab using CartPole as an example.

It covers:

• Defining the robot in MuJoCo XML.
• Setting up the task (observations, actions, rewards, etc.).
• Training with PPO.
• Viewing the results.

This demo is based on the "Create a New Task" documentation.

Tested in Google Colab. Training works, viewer launches. If needed, I can update README to link to it.

Thank you for developing such a great product!