add vignettes sections on OSN dataset retrieval/caching, `reticulate`-based functionality etc.

[HelenaLC]

• programmatically list datasets available through Bioc's OSN, demo retrieval and caching with BiocFileCache()
• demo spatialdata-io for reading vendor data, writing to SpatialData .zarr, and reading into R
• demo how to align elements to an (identity) space in Python before going into R