Add Probability Mass Functions (PMF) notebook

[Haleshot]

📝 Summary

Followed the PMF chapter from Stanford's "Probability for Computer Scientists" course to create a notebook on Probability Mass Functions.

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

• Clear definitions and properties of PMFs with mathematical notation
• Visual examples of PMFs for dice rolls with proper explanations
• Interactive exploration of common discrete distributions (Bernoulli, Binomial, Geometric, Poisson)
• Comparison between PMF and CDF with visual explanations

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.

[akshayka]

@Haleshot, this is excellent! Nice work :)