Minor Reorganisation (title and autodiff)

[mmcky]

Feedback from @jstac

2. For now I suggest that we keep the JAX lecture series separate but rename it to `GPU computing for computational economics'.

• [ ] rename lecture series to new title and add brief description on intro.md

At the start we can explain that we're going to do all our GPU computing using JAX.

This lecture could be shifted to the Python programming series, after we introduce JAX: https://jax.quantecon.org/autodiff.html

It's not GPU specific, after all.

• [ ] shift autodiff to lecture-python-programming.myst

[jstac]

Thanks @mmcky :-)

Comments welcome.