Switch docs version via URL

[octopuss]

This is related to #13. We should somehow differentiate between different versions of documentation via url path variable.

The question is, if user should be redirected to current version right after entering the page (e.g. / --> /en/v2/) or we should always use two urls for current/latest documentation (this might be a SEO issue).

[borekb]

We don't need this for v1, it's just a possible improvement in the future.

[octopuss]

For me it was just reminder issue. It is ok to be in Improvements milestone.