Add a tutorial for Gaussian Processes with GPJax

[rlouf]

We should add a step-by-step example of how to use Blackjax to sample Gaussian Processes models. Part of a series of tutorials that show how Blackjax integrates with the broader JAX ecosystem.

[thomaspinder]

Not sure how/if I can be helpful here, but please let me know if there's anything I can help with.

[rlouf]

Thanks! We're looking for something simple like your classification example (but only with the inference part). It's essentially to signal potential users that they can use Blackjax with GPJax easily.

I also noticed there is some variational inference functionality in your library and I wonder if at least part of it could be handled by Blackjax now that we're also moving in the VI direction.