Add spatial tutorials

[grst]

There should be tutorials giving a general introudction to

• spatial anlaysis with squidpy
• how to use spatialdata

[grst]

• [x] squidpy: https://squidpy.readthedocs.io/en/latest/notebooks/tutorials/tutorial_vizgen_mouse_liver.html

• [x] subcelllular analysis with bento: https://bento-tools.readthedocs.io/en/latest/tutorial_gallery/Main_Guide.html

• [ ] There's a bunch of spatialdata tutorials, but I'm missing some basic introduction to the data structure (what are the different slots, how to get data in etc.)

[LucaMarconato]

You are right, I add the creation of this tutorial to my todo list.

[LucaMarconato]

Tutorials will be ready by the scverse conference, after these they can be added to the website.