Option to choose documentation version at top of static website

[robertapplin]

Description
When users search for mantid documentation, they will often be directed to documentation from an old version of Mantid. We want to make it more obvious that they are on an outdated documentation page. This problem can cause users to thing our documentation is out-of-date, or not working according to our documented instructions.

Solutions considered
A banner at the top of old website versions with a link to the latest stable and latest nightly documentation.

A combobox at the top of our documentation pages which allows people to choose a version of mantid more obviously than needing to change the URL.