Add tutorials covering SpatialData, SnapATAC2, pertpy, LIANA+, scirpy BCR and decoupler

[Zethson]

Summary

Adds 10 tutorials. Part of scverse/scverse.github.io#142.

• SpatialData: Introduction to SpatialData (Data structures), Reading Xenium data (Spatial)
• SnapATAC2: Standard scATAC-seq pipeline (ATAC-seq), Single-cell multiome analysis (Multimodal)
• pertpy: distances, perturbation spaces, perturbation efficacy in pooled CRISPR screens (scRNA-seq)
• LIANA+: Cell-cell communication (new category)
• scirpy: B-cell receptor analysis (Adaptive immune cell receptor)
• decoupler: Single-cell enrichment analysis (scRNA-seq)

Cell-cell communication is a new value in the primary_category enum, so schema.json and categories.yml change too. Multimodal was already defined but had no tutorials.

Authors are taken from each notebook's upstream commit history. Icons come from the ecosystem-packages registry, except LIANA+ (own repo logo, resized to 446x512) and decoupler/scirpy (icons already in this repo).