[CI] Add CircleCI workflow to build docs for preview
This PR adds a CircleCI workflow to build the documentation and store it as an artifact so that we can preview it and verify it's rendered properly.
The built documentation:

https://30100-155220641-gh.circle-artifacts.com/0/docs/_build/html/index.html<|||||>How to view the built documentation:

![doc](https://user-images.githubusercontent.com/17039389/79070944-ba335b80-7d13-11ea-9d19-71e8fc278226.gif)<|||||>@LysandreJik this might interest you!<|||||>@sshleifer Thanks for the comment!

[The CircleCI doc](https://circleci.com/docs/2.0/artifacts/) says:

> Artifacts are stored on Amazon S3 and are protected with your CircleCI account for private projects. There is a 3GB curl file size limit. **Artifacts will be accessible for thirty days after creation**.