Feature Request: Allow Traits to Subtype Arbitrary Type

[SBuercklin]

It would be helpful if we could have traits subtype some type we define, rather than Any. This allows us to filter traits by supertype when generating documentation.

For example, say I want to make an Animal interface, and then I have Fish and Bird traits as well each with their own interface that is independent of Animal. I add a docstring to each of these Animal, Fish, Bird traits to document the interface they must satisfy.

Now I want to make a Habitat interface, and within that I have Water and Tree traits which each include their own interfaces. Again, I document the interface in the trait itself.

I would like to generate a Documenter.jl page which includes all of the Animal interfaces, and another page which includes all of the Habitat interfaces with @autodocs. Currently, we can do this by manually including the docstrings for each trait or restricting trait implementations to particular files, but it would be helpful if I could filter the trait types according to the Julia type hierarchy to generate my interface documentation.

In the end, I'd like to generate an Animals interface page via

@autodocs
Module = [Foo]
Filter = t -> typeof(t) === DataType && t <: Foo.AbstractAnimal

where AbstractAnimal <: Any is an abstract type I create to make sure that {Animal, Fish, Bird} <: Abstract Animal are all independent types in the type tree

Relevant link to the Documenter.jl docs

[SBuercklin]

Closing as this is already implemented with

@trait Animal as AbstractAnimal

Docs cover it here