Fix errors and modernize keras and jax_nn lectures (#302)

* Fix errors and modernize keras and jax_nn lectures

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Fix smart quotes in jax_nn.md causing CI lex errors

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>

Author: 3 people