Allow version-specific documentation starting with v0.4

With the upcoming release we will need to have documentation for different versions of Stratosphere. @rmetzger already suggested to have this, when I did the initial overhaul of the documentation.

With this PR, the documentation is under `/docs/0.4` instead of just `/docs`. The docs root and `/docs/latest` redirect to the latest version, i.e. the upcoming 0.4.

Preview under http://uce.github.io.

I suggest to start with 0.4 as the initial open source version for which we provide documentation, as porting the old documentation will be too much work right and we will probably suggest to anybody who uses something older to move to 0.4 anyways.