Fix doc preview in PR CI

[lucyleeow]

• Update names downloading artifacts: we changed workflow name and build dir from 'docs' to 'html' -> this updates this and updates the images to match new names
• Adds more info on difference between local vs PR preview and circle CI vs artifact download preview.

[psobolewskiPhD]

Wait, that's the contributing documentation guide, so for this repo.
In this repo, on PRs, we still have the action that make the artifact. I think it's useful to check it -- it's more of the ground truth than the CircleCI because in principle it's what gets deployed to gh-pages, same runners, same workflow now (since it was unified with all the consequences).
So I'm not sure I like totally deleting the info on downloading the artifact...

(That said on napari/napari it's just circleCI now, which is fine I think because most docs things will go through this repo.)

[lucyleeow]

No problem, we can document both. Still need to update the docs as we get to the artifact a different way now.

[lucyleeow]

I've updated everything (since we updated the CIs things have changed names) - hopefully its okay, I'll come back and double check later!

[lucyleeow]

🤦 oh man I just realised that you could still download the artifact and the old docs described how to do both and were fine really.
Oh well updated names and added more info

[psobolewskiPhD]

Thanks for updating things!

[melissawm]

Looks great to me, thanks @lucyleeow !