Versioning in documentation

Steps:
1. Follow the guide [here](https://microsoft.github.io/FLAML/docs/Contribute#documentation) to build the documentation website locally. After this step, one should see a `reference` folder created under `website/docs/`.
1. Follow https://docusaurus.io/docs/versioning to create a versioned doc. Check https://docusaurus.io/docs/docs-multi-instance to separate the parts that need to be versioned and the parts that don't.
1. Test the change locally to verify it works as expected.
1. Make a PR to check in the versioned doc.

The above steps should be repeated every time before a major new release which modifies user-facing APIs.