Title: [Docs] Update outdated book prices and verify resource links

[AG141293]

Problem

Several book prices listed in the curriculum appear to be
outdated. For example:

• "Stats in a Nutshell Book $46" — current price differs
• "Think Stats Book $34" — now available free online
• "Programming Collective Intelligence Book $46" — price changed
• "Algorithm Design, Kleinberg & Tardos Book $125" — price changed

Suggested Fix

1. Update all book prices to reflect current prices
2. Mark books that are now freely available online with
   a 🆓 or "Free" tag
3. Add direct links where free versions are available

Why This Matters

Students relying on this curriculum for budgeting their
learning resources need accurate pricing information.
Outdated prices may discourage learners who think
resources are more expensive than they are.

References

• Think Stats free version: https://greenteapress.com/thinkstats/
• Think Bayes free version: https://greenteapress.com/thinkbayes/