Add notebook for Variance

[Haleshot]

📝 Summary

Followed the Variance chapter from Stanford's "Probability for Computer Scientists" course.

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

• Clear definitions and properties of variance with mathematical notation
• Step-by-step calculations of variance for dice rolls w/ verification of key properties
• Interactive visualizations showing how variance affects probability distributions
• Visual demonstration of the 68-95-99.7 rule for normal distributions
• Explanation of standard deviation as a more intuitive measure of spread

Checkout the notebook here:

📋 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.

[Haleshot]

@koaning Used wigglystuff's Tangleslider here! (Felt this would be better for a more inline view).