Use new numpy random number generator in docs example

[schlich]

Hi folks, just getting started with datatree and hoping it's the solution i've been searching for :) Just opening a very minor PR I noticed while going through the intro tutorial hands-on. I usually keep my Ruff linting settings pretty strict and it chimed in with the NPY002 linting rule. I have a patch branch for on my fork that addresses this change, and I'd be happy to open a PR with a little more contributors' guidance and encouragement 🚀

[TomNicholas]

Great! Please let us know any other thoughts you have as you explore the package :)

I'd be happy to open a PR with a little more contributors' guidance and encouragement 🚀

Definitely PRs welcome! Most things from xarray's contributing guide apply here also. Feel free to ping me with questions.