Add SpaceM datasets

[aeisenbarth]

This PR adds two SpaceM datasets to the list of example datasets.

Additionally, I suggest a notebook that visualizes and explores the contents of the dataset. Descriptions in the notebook may still be expanded a little. Also, I can remove the analysis section if it is not relevant, and just keep the visualization with spatialdata-plot.

Work still needed:

• Transfer datasets to storage
• Merge of Add SpaceM datasets giovp/spatialdata-sandbox#47
• Update 4 URLs (2× .zarr.zip, 2× .zarr/) and file sizes in README.md

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[LucaMarconato]

Thanks, I updated the URLs.

Notes:

• I skipped the S3 URLs from the README because the data is still not processed and uploaded by the CI. I will do this once the problem with . in the name is addressed.
• I put the URLs for the full data in the README, and I added both the URLs for the full and small data in the notebook.
• I would not add a notebook also for the other dataset, I think one is enough.
• In the small dataset I could not plot the UMAP for "replicate", CONDITION_COL since they were not present, so I removed them.
• I added the notebook to the visualization notebooks in the docs.

Todos:

• close Naming constraints break compatibility with existing datasets spatialdata#624 before merging this PR.
• (andreas) please can you add a small icon for the docs under _static/img/spacem.png?
• (andreas) also, please add a citation file to the README, if available
• (luca) before/after merging this PR, include the notebook in the CI.

[LucaMarconato]

@aeisenbarth going back to this, do you have a citation for the datasets? For the moment I will link the README files that you added in spatialdata-sandbox. If you think that the info contained there is enough we can keep it like this.

[LucaMarconato]

Thanks @aeisenbarth, I reviewed the PR and closed the latest open points. Merging now!

[LucaMarconato]

(luca) before/after merging this PR, include the notebook in the CI.

Addressed here: #129