add numpy version warning to documentation

[rxba]

Motivation and Context

Release of numpy==2.0.0 causes many Open3D features to segfault, which I saw will be fixed with the pybind update in #6874 .
Due to only requiring numpy>=1.18.0 in the Open3D<=0.18.0 releases, new Open3D installs will also install numpy==2.0.0, and thus break a large part of the functionality.
I think it's important to communicate this to users, although I'm unsure if this is the right place and way to do so, especially since this documentation will also only update for the next release of Open3D.
I hope this feedback is helpful, thank you!

Description

Add warning about numpy versions to the "Getting Started" documentation.

[ssheorey]

Hi @rxba the current requirements.txt lists (merged a few days ago):

Open3D/python/requirements.txt

Line 1 in 2c8bb60

numpy>=1.18.0,<2.0.0

If you install the latest devel wheels (from github releases), pip will install numpy 1.x. If numpy 2 is installed, pip will complain.

[rxba]

Hey @ssheorey , I'm aware of this change. My point was that the majority of users will install a previous release instead of the current devel wheels, and thus most likely end up with a non-functioning version of Open3D without knowing it and without much feedback as to why their code is not working, due to the segfaults.

If you feel like this is a non-issue, feel free to close this PR.
Thank you for your work on this great library!

[ssheorey]

Makes sense. Thanks for the PR!