Use S3 for documentation previews (#60711)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60711
We already build the docs on each PR, this adds a step to push the relevant folder of the docs (we build the entire website for pytorch.github.io which clocks in at around 500 MB, but we really only need the "master" docs, not every version. The master docs by themselves are around 50 MB which is more reasonable). It uses the same S3 bucket as the artifacts but places the items at the `pytorch/pytorch/pr-previews/<pr number>` prefix. The bucket has a rule to expire resources in that prefix after 1 month.
On the AWS side the bucket has static hosting enabled with CloudFront directing to the docs preview prefix, so you can see the output at `https://d28slxzaq48q8t.cloudfront.net/<pr number>/`, e.g. https://d28slxzaq48q8t.cloudfront.net/60711/. For advertising we could link this on the HUD PR page as well as in the Dr. CI comment. We could add a CNAME on CloudFront to make this be `pr-preview.pytorch.org/<pr number>` or something but having random PRs be able to host content on the pytorch.org domain seems sketchy.
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D29398818
Pulled By: driazati
fbshipit-source-id: 24032854d83815853b3650d8e54f60b684707f76
