Improvement to the docs

[LucaMarconato]

Routinely listing improvement we can make to the docs:

• add rasterize_bins() example in Visium HD notebook;
• show how to use get_element_instances() in the annotation notebook.

[LucaMarconato]

• make a notebook showing how to add an existing segmentation to an object; give just an introduction and refer to SOPA tutorials for more in-depth analyses
• put a link to the spaitaldata-xenium-explorer notebook and a link to the Squidpy Xenium notebook in the xenium spaitaldata notebook

[LucaMarconato]

• add an example of transform() in the transformations notebook (suggested by "liz-is" via Discourse)

[LucaMarconato]

Using less and simpler datasets, tracked here #700