Properly version documentation

[wbnns]

Set up documentation to accommodate different versions of Core.

[cornelius]

I think this can be done with creating branches for the different versions. So with each release we would create a corresponding branch in the documentation repo. For example readthedocs can then show the different versions with its standard version switcher mechanism.

[wbnns]

Ok, was just checking this out in case you haven't seen it: https://docs.readthedocs.io/en/stable/versions.html