Document which operations are required for a SparseArray eltype

[LilithHafner]

getindex on structural zeros uses zero(T).
setindex!, dropzeros!, and == use iszero(T).

It would be nice to know which operations are required for full functionality for SparseArrays, and what subset of that functionality is available if some or all of those operations are missing. For example, do I need to define zero or iszero on my custom type T to support sparse matrix multiplication wth eltype T?

[ViralBShah]

This has mostly evolved over time. So it would be great to document what we need.