Provide ready access to documentation of alternative versions

[MatthewHambley]

Each releases documentation is stand alone, which is what we want, but there is no easy way to switch between versions.

[MatthewHambley]

Having done a bit of reading it seems like the capability to do this using the Sphinx theme we currently use (RTD) is deliberately disabled. This is a feature, not a bug, for the RTD developers.

We will need an alternative solution.