Ability to import a notebook.

[GordonSmith]

Currently its possible to export a Notebook as a JSON file via the API (for example:
https://api.observablehq.com/document/@observablehq/chart-starter-recipes)

It would be great if we could also import a complete notebook (with a subset of the exported JSON, or just an Array cells).

[tomlarkworthy]

its a poor substitute but I made https://observablehq.com/@tomlarkworthy/paste-codegen which allows programatic generation of a pastable notebook