separate data structures from other packages

[grst]

As discussed, we should separate data structures from other packages on the website.

Also

• we should add spatial data
• I think it could make sense to link to the airr extension of anndata (and similar specifications for other modalities, if they exist)

This page should be the one-stop-shop on "how to represent data type X in scverse data structures".

CC @Zethson

[Zethson]

I actually think that we're kinda doing that already (or now?) so I'll close this issue.