Adding plotting specific documentation

[LucaMarconato]

Comment originally from @MeyerBender, thanks for the feedback!

documentation: in general, I think it would be good to have one or two notebooks that exclusively focus on visualization. Right now, it is very hard to find this in the spatialdata docs. Maybe something similar to this, which shows off the most important plotting functions in spatialproteomics.

Currently we have plotting notebooks but hidden under the "Technology-specific" section in the docs. And we have another notebook in a separate repo (=not really discoverable).

Improving the docs with a visualization-specific notebook that is easily discoverable would be valuable.