Use Literate.jl for long-form documentation, including Jupyter notebooks

[epatters]

We now have several Jupyter notebooks providing long-form, tutorial-style documentation. They do not version control cleanly, and it is easy to forget to update them, because executing them is not part of the build process.

I think Literate.jl, which is reminiscent of R Markdown, could provide a good solution to both problems. We can generate and execute Juypter notebooks during the build process, and also generate Markdown files suitable for Documenter.jl.