Improve documentation

[thomasgudjonwright]

To add:

• API.md
• example.md
• Update README
• more...?

[willtebbutt]

On this, it would be good to get the examples set up using Literate.jl, in the same way that we do in e.g. KernelFunctions.jl, so that they're built when the docs are built (meaning we find out if they break when we make changes), they can be embedded in the docs, and the code to generate them is reviewable.

Definitely doesn't have to be done today.