Upload artifacts in PR build

[eteq]

• [x] Check the box to confirm that you are familiar with the contributing guidelines and/or indicate (check the box) that you are familiar with our contributing workflow.
• [x] Confirm that any contributed tutorials contain a complete Introduction which includes an Author list, Learning Goals, Keywords, Companion Content (if applicable), and a Summary.
• [x] Check the box to confirm that you are familiar with the Astropy community code of conduct and you agree to follow the CoC.

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).

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[adrn]

@eteq This is fine with me! Can you remove the noop commit and then we can merge?