Set up Documenter

[simonbyrne]

It will require setting up the necessary documenter keys, unfortunately I don't have access to the repo.

[simonbyrne]

There's more that can be added, but this feels like a good start.

[piever]

Now that it is documented, it feels really odd that the API to map a function to a (Named)Tuple type returns a type or an object depending on an underscore... Even though it's internal, it may be worth to rename it (don't have great ideas though), but that should probably be done in another PR.

[piever]

Thanks a lot, this looks great! I've added the documenter key. From what I understand, push_preview doesn't work because the PR comes from a fork and not from a branch of the main repo, but I've checked the docs locally.

[piever]

I wonder whether we should keep the README as is, or maybe it could be moved to a "tutorial" section of the docs. Somehow, I think that it's helpful to have a quick way of learning how to use the package without going through the API.