Add random-variables notebook

[Haleshot]

📝 Summary

Followed the Random Variables chapter among many other sources, got an initial draft up and running.

Appropriate markdown + LaTeX used to explain the concept.
Added:

• Function-called examples to explain the concept and functions implemented.
• Interactive example relevant to the topic at hand.
• Practice exercise + Test your Understanding section.

Checkout the notebook here: https://marimo.app/https://github.com/marimo-team/learn/blob/13db4aca2ebfa4d1f3c5b9a047babbd64b17d370/probability/09_random_variables.py

📋 Checklist

• I have included package dependencies in the notebook file using --sandbox
• If adding a course, include a README.md
• Keep language direct and simple.

[akshayka]

Made just small edits. Moved the definition of distributions to above the interactive example (where the distributions where used). I really like the interactive example!