Upgrade notebook Gaussian Process to V4 (#404)

* ⬆️ upgrade nb to V4

Co-authored-by: Ana Rita Santos <[email protected]>

* 👌 updates due to review comments

* 👌 sampling with multivariate_normal from xarray_einstats dev

* 📌 set ablog version

* 🍻 🐛 fix the pinning

* 📌 pin sphinx and sphinx gallery

* 📝 add watermark header

Co-authored-by: Ana Rita Santos <[email protected]>

Author: symeneses