Don't build Quickstart with docs

[sethaxen]

Currently several PRs are halted because the documentation build stalls when building quickstart. It's hard to tell what the culprit here is, since Documenter doesn't output very detailed debugging logs. It would be good to pre-build Quickstart and periodically update it manually, similarly to how ArviZ does it.

Unfortunately, Documenter has no way to integrate Jupyter notebooks or pre-built HTML files, so for the moment, this will probably require linking to a Jupyter notebook in the repo or in nbviewer.

[sethaxen]

Alternatively, we can use the datasets packaged with ArviZ to showcase the plots in a quickstart. From there, we can link to individual notebooks in an examples folder for Turing, CmdStan, and Soss. This way we can avoid some of the compatibility clashes that sometimes arise between these packages.

I'll write the examples so that I can use Literate.jl to turn them into either notebooks or markdown, but this will still be done locally and periodically updated instead of using CI.

[sethaxen]

Closing in favor of building the docs on Azure (#48), which can handle long builds.