Add doc versioning to support vNext

[mmacy]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• This is a feature request for the Python library

Describe the feature or improvement you're requesting

Hook up doc versioning so we can stage docs for yet-to-be-released library versions as vNext (and flip between vLatest and vNext or even retain older versions).

Additional context

No response

[mmacy]

Fixed in #11