[harrison_kreps] Update lecture

[xuanguang-li]

This pull request updates the Harrison-Kreps model lecture to use JAX and improves code consistency and formatting throughout the document.

• Replaced numpy (np) and scipy.linalg (la) imports and usages with jax.numpy (jnp) equivalents throughout code cells, but kept for and while loops for simplicity.

• Formatting and terminology improvements: standardized section and subsection titles, and improved consistency in emphasis (e.g., italicizing instead of bolding, fixing typos in comments and variable names).

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📚 Changed Lecture Pages: harrison_kreps

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[jstac]

Thanks @xuanguang-li , much appreciated.

I've reviewed the PR.

Actually I don't think switching to JAX is justified in this lecture.

In the computations of the stationary distribution at the start, we will be using NumPy internally, since that's all qe.MarkovChain can work with.

In the later computation, with the for loop, JAX is probably slower, since the array computations are very low dimensional and the main task is iterative.

I think it might be best to close this PR.

@HumphreyYang or @mmcky, what do you think? If we do close this, let's tick it off or remove it from the meta issue, so we know not to try again.

[HumphreyYang]

Many thanks @jstac, I agree!

@xuanguang-li also has some other valuable edits in this PR so I can revert the JAX edits and keep others.