docs(backlog): delete the DataFusion function catalogue

doc-1 was the generated output of scripts/gen_datafusion_catalogue.py, which
this branch removes -- an exhaustive function surface for an engine that no
longer exists.

doc-2 linked to it, so that reference is rewritten rather than left dangling,
and doc-2 gets a stale banner: it was written for the deleted line, its
canonical decision record (decision-4) went with the decisions, and the sklearn
approach is being reconsidered from scratch in DRAFT-20. Its transformer
priority list is the part still worth having.

Author: ahrzb