Add image diffs against previous build to CI results

[QuLogic]

Unfortunately, from what I've read, you can't upload an image from Actions, so you still need to download the build artifact to see the diff, but it's one less step to do yourself manually.

[QuLogic]

you still need to download the build artifact to see the diff

i.e., it's part of the build Artifact at the bottom of the Check: https://github.com/matplotlib/cheatsheets/actions/runs/4552134723?pr=130

[dstansby]

I've merged master here to fire CI since the old artifacts had expired.

[dstansby]

Would be nice to have the diffs in a more discoverable directory? It took me a while to work out they were in docs/_build/html. Also there seems to be non-zero diffs which doesn't seem right? e.g.:

[QuLogic]

OK, I moved it to a separate artifact to make it easier to find. Now with artifacts v4, we can generate a URL, and with build step summaries, we can put that all on the summary.

So you can see the summary dropped here: https://github.com/matplotlib/cheatsheets/actions/runs/7706808370#summary-21002939154

[story645]

Is there a way to have the diffs show up as a standalone check/see which check in the actions run is the diff check?

[QuLogic]

I think the only way to do that would be to move it into a separate job that depends on the build one, download the artifacts from it, and run comparison from there.

[dstansby]

I think worth getting this in as is - I opened #143 for making the diff check a seaparate run.