Visual clue to indicate old, current and future documentation

[ljacomet]

While Ehcache 3.0.0 has been released, with all its documentation under documentation/3.0, the upcoming 3.1.0 release will have a different documentation as clustered is added to it.
I believe the idea is to have that documentation under documentation/3.1.

Whenever we reach that point, it would be great to have a visual way on every page documentation to indicate which version is current and which is for a previous version.

And by the way, if we were to do that, it would be pretty easy to extend the visual clue so that we could publish documentation for a future version without risk of confusing users.

Opinions and suggestions welcome!