Re-enable PR website Preview

[szha]

Description

It would be great to re-enable the preview capability for the website and API doc in PR for the purpose of quality control. We used to have this feature and it was removed as part of restructuring the website. Specifically, it should support:

• Uploading the built preview doc in CI into a static hosting place and making it publicly available.
• Adding a link to the preview site as a comment in the PR.

You can see examples of this in action in autogluon/autogluon#548

[szha]

In the autogluon case, we used S3 for the static hosting, and I created the staging bucket for this purpose. The bucket is set to have a lifecycle configuration that automatically deletes files older than 30 days so that the file space doesn't blow up

[ys2843]

For MXNet website, we need to host the preview on an Apache v2.4 server in order to make all the redirect work the same.

[szha]

@ys2843 can this be applied onto s3, or otherwise can we get rid of the redirection?

[szha]

@ys2843 @sandeep-krishnamurthy please treat this task as high priority. now that we have a stable version as the default on the website, the website fixes are not nearly as urgent as being able to preview the website changes automatically in each PR. thanks.