New SUBTYPES exported key

[Datseris]

I am often finding myself in the situation where I dewsign an interface, and the algorithm-deciding arugments of the interface are all subtypes of a main abstract type. Like

abstarct type A end

struct Method1 <: A end
struct Method2 <: A end

In this case, I always cross-reference the subtypes of A in the docstring of A, as the same docstring describes the interface. It would be great to have a SUBTYPES key in the extensions here, so that it expands all subtypes of A into a Documetner [@ref] wrapp.