Guidance on docs versioning

[carljparker]

I've received feedback that the versioning UI is both not obvious enough and also that it is not clear what it means. Adding some copy to the page to clarify.

[mthrok]

I am not sure the effectiveness of this approach. I think what we should do is to either warn readers on all the pages like done in boost https://www.boost.org/doc/libs/1_79_0/ or old Python https://docs.python.org/3.6/library/index.html, or/and make the version so noticeable like currently supported Python docs does. https://docs.python.org/3/library/index.html