Setup documentation

[tkf]

There is a setup in .travis.yml but it is not deployed properly because DOCUMENTER_KEY is not setup.

@c42f Can you set configure it for Travis CI (or GitHub Actions)?

https://juliadocs.github.io/Documenter.jl/stable/man/hosting/

[c42f]

For sure, will do. I'll read up on Travis vs github actions. Do you have any preference between those for documentation at this stage?

[tkf]

I think GitHub Actions is a bit better because:

• it supports push_preview https://juliadocs.github.io/Documenter.jl/stable/lib/public/#Documenter.deploydocs
• we keep colorful test log in Travis

But I don't mind using Travis at all, especially since you already half setup Travis.

[c42f]

Good call, GitHub Actions seems very slick, should be working now I hope (#14).
I've turned push_preview on which is pretty cool.