Support Sourcegraph notebooks on docs site

[jtibshirani]

Our developer docs contain some code walkthroughs that would work really well as Sourcegraph notebooks. For example, I tried converting the Life of a search query docs to a notebook and found it more readable. It'd be great if docs.sourcegraph.com could show notebooks like this. If we had notebooks throughout the docs, maybe more users would learn about them and see how useful they are.

Currently, the way to do this would be to create a public notebook on https://sourcegraph.com/notebooks, and then embed that in an iframe in a docs page. But this would mean separating out important documentation from the sourcegraph/sourcegraph repo where it currently lives. It seems best to keep all the docs in one place, living alongside the code.

Maybe this would be a new notebooks feature? You could just point to the URL of a running Sourcegraph instance, plus the location of a .snb.md file, and it will render it?

[jtibshirani]

CC @novoselrok since I heard you worked on something similar -- please ignore if this isn't right!