Complete JAX intro lecture: add pure functions content and fix errors

- Add comprehensive "Random numbers and pure functions" section
- Demonstrate NumPy's impure random number generation vs JAX's pure approach
- Fix spelling errors: discusson→discussion, explict→explicit, parallelizaton→parallelization, hardward→hardware, sleve→sleeve, targetting→targeting
- Fix grammar: "uses use"→"uses", "short that"→"shorter than", "function will"→"functions will", "Prevents"→"Prevent"
- Fix missing jax. prefix in random number examples
- Improve clarity and consistency throughout

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jstac