Improvement to the docs

[LucaMarconato]

Routinely listing improvement we can make to the docs:

• [x] 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 https://github.com/scverse/spatialdata/issues/700