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

[LucaMarconato]

As suggested here from a user scverse/spatialdata-plot#371 (comment):

• explain, in the optimization notebook of spatialdata-plot, that the data needs to be saved to disk to have the multiscale, chunked representation optimization enabled
• make the spatialdata-plot and napari-spatialdata notebooks also available in the respective repos. Also, link them all in the spatialdata docs, in a clear section. Right now, the spatialdata-plot notebooks are stored in spatialdata-notebooks and shown in spatialdata but not in spatialdata-plot. Instead, the napari-spatialdata notebooks are stored in napari-spatialdata and shown in napari-spatialdata but not in spatialdata.

[LucaMarconato]

• add docs for get_element_annotators().
• Consider moving get_annotated_regions() and get_element_annotators() to the same place, and make names more uniform.