Simplify navigation between Qt versions in API documentation

Business justification: Would significantly speed up doc navigation in many cases, which results to improved developer experience. 

Effort estimate: 

Description:

User story: As a developer, I want to be able to easily move from one version of our documentation to another.

When browsing Qt's API documentation, by e.g. following internet search (Google) links, you land on a given page. You might later realise that this is not the documentation for the  version of Qt you are currently using, and you need to manually navigate to the relevant page for the correct version.

In internet search results, the links for the version you are using might be far down in the search results, and the easiest solution would often be to navigate to the current version and jump to the version you want directly from there.

An added advantage would be that this would make it easier to see feature differences between versions, which might improve adoption of newer versions and as such, sales.

Example Qt documentation page: http://doc.qt.io/qt-5/q3dbars.html

Example from the Django documentation, where this version navigation is supported: https://docs.djangoproject.com/en/2.0/ref/views/