Adding more examples to the gallery

[kolibril13]

Hi there,

In this pull request, I’ve added some new examples and made a few improvements.
Fistly, I added a new notebook, 01_download_all_data.ipynb, which handles downloading all the assets.
Previously, each notebook contained commands like:

!mkdir -p ../images/
!wget https://github.com/niivue/niivue/raw/main/demos/images/narps-4735_50GV-hypo1_unthresh.nii.gz -P ../images/
!wget https://github.com/niivue/niivue/raw/main/demos/images/narps-4965_9U7M-hypo1_unthresh.nii.gz -P ../images/
!wget https://github.com/niivue/niivue/raw/main/demos/images/mni152.nii.gz -P ../images/

Having these commands at the top of every notebook felt cluttered, and running “Run All” would repeatedly re-download the assets, which is not efficient.

For better development convenience, I suggest we remove these headers from all notebooks and centralize the asset download in 01_download_all_data.ipynb.

Additionally, I’ve reorganized the example image folder, so the paths no longer need the ../ prefix. For instance, ../images/mni152.nii.gz is now simply images/mni152.nii.gz.

The notebook meshes_(GIfTI, FreeSurfer, MZ3, OBJ, STL, legacy VTK).ipynb does not work yet because nv.setMeshShader is not yet implmented

And the notebook trajectory.ipynb does not work yet because the fiber feature is not yet implemented yet (probably).

Therefore, I'm making this a draft pull request