Add examples for advanced data passing, e.g. how to pass `x_attr/y_attr`

[MUCDK]

          Can we create an issue for this?

Originally posted by @michalk8 in #633 (comment)

[MUCDK]

also mention that set_xy/x/y, and set_graph_xy exist, and point to the relevant examples.

[MUCDK]

main message: "show where we can fetch data from our anndata object"

• fetch data from .X (e.g. if you wanna have raw counts)
• fetch data from .layers (e.g. if you wanna have normalized counts)
• fetch data from .obsm (e.g. if you wanna have PCA, but please do example with something else)

Use a FGW problem, and take different embeddings for all of them, e.g. with the MappingProblem:

• x: normalized spatial (no callback here, but something pre-computed (but obv. link to callback))
• y: PCA of all single-cel genes
• xy: 50 most highly variables genes which are log1p normalized