Notebook preview for PRs

[greimel]

It would be really cool, if notebooks were also built for PRs. Though rather than uploading them to gh-pages it would be nice to just upload them as artifacts for a preview (e.g. using actions/upload-artifact@v2.)

I couldn't figure out how to do this in one action, so that gh-pages and the artifact preview share the cache correctly.

[sosiristseng]

Working on this, I found workflow-artifact-pull-request-comment helpful to leave a link to the artifacts in the comments.

As to cache sharing, the PR branch could read the base/default branch's cache but not the other way around, AFAIK.