Add pedagogical notebook: PageRank with NetworkX (closes #13)

[FatimaIsmailHere]

This notebook explains the PageRank algorithm using NetworkX.

It covers:

• Intuition behind PageRank and the random surfer model
• Graph construction and visualization
• Computing and interpreting PageRank scores
• Comparing with degree and betweenness centrality
• Damping factor experiments
• Real-world example using Zachary's Karate Club

Contributes to #13

[MridulS]

This reads too much as generated by an LLM and we already have an open PR #111 covering pagerank already.