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This PR adds the upload artifact stage to the PR workflow (but not the actual deploy, of course), so that we can see the rendered version of a notebook before it's merged. This could be thought of as a first step of #532 (although that needs more before I think we would consider it closed - see the discussion there).

Note the second commit here should be removed before merging - that's just to trigger the actual build process on one of the notebooks. So the intent is to back out that last commit before merging (although it leaves an unnecessary comment).