datatree: Use new numpy random number generator in docs example

What is your issue?

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 🚀

Originally posted by @schlich in https://github.com/xarray-contrib/datatree/issues/290#issue-2027711482

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.

Originally posted by @TomNicholas in https://github.com/xarray-contrib/datatree/issues/290#issuecomment-1842136357

Start with the datatree intro tutorial example mentioned in the issue and compare it with the contributor's patch branch. Check Ruff's NPY002 guidance, update the example to use NumPy's newer random-number generator, and verify the documentation example no longer triggers that lint rule.