[skip ci] Use Stanford Cars mirror to fix CI docs build

[guberti]

As part of gallery/how_to/work_with_microtvm/micro_train.py, we download a part of the Stanford Cars dataset from https://ai.stanford.edu/. Unfortunately, https://ai.stanford.edu/ is currently down, with no indication of when it will be back up. This PR provides a temporary CI fix by switching this tutorial to use a mirror link. Note that the link is not an exact mirror (containing all files, not just the train ones), so a few other lines had to be adjusted too.

cc @Mousius @areusch @driazati

[mehrdadh]

added skip-ci to unlock the CI quickly.

[guberti]

I'm surprised http://ai.stanford.edu/ went down - Pytorch and Tensorflow both use this link for their datasets packages, so it seems to be trusted. I'm not sure what the best long-term fix for this issue is - see the discussion on #11811 for more.

[driazati]

lgtm assuming it works locally

[guberti]

Can confirm the fix works locally. You can try it yourself on Google Colab at https://colab.research.google.com/drive/1ycMRXm3ulghiBKsdrp1wiKdFpmXNej9o?usp=sharing (click "Run All").

[driazati]

also fyi to skip Jenkins [skip ci] must be prepended to the PR title, but to skip GitHub actions jobs it must be on the commit message subject (so to skip CI for this commit you could git commit --amend and add [skip ci] then re-push)

[guberti]

also fyi to skip Jenkins [skip ci] must be prepended to the PR title, but to skip GitHub actions jobs it must be on the commit message subject (so to skip CI for this commit you could git commit --amend and add [skip ci] then re-push)

Good to know, thanks!