Multi version selector drop-down

Can you please provide some directions on how to enable multi-version drop-down selector instead of version subtitle?

I was going over this suggestion - https://github.com/squidfunk/mkdocs-material/issues/770#issuecomment-384556794, but not exactly sure how to apply it on a `windmill`.

Thanks!

Compare the windmill theme's current version-subtitle behavior with the multi-version selector suggestion in the linked mkdocs-material issue and comment. Identify where theme usage or configuration is documented, then make the selector setup understandable for windmill users and verify that the resulting directions are complete.

Written by the indexing model from the issue text.