No easy way to switch between versions of the specification on website

[dstansby]

From the web pages of one version of the specification (e.g., 0.5), there is no easy way to switch to another version of the specification (e.g., 0.4). This would be a nice feature to add - often I find myself reading one version, and wanting to switch to another to compare.

[jo-mueller]

@dstansby There's a good chance that we are moving on to readthedocs for versioned releases. It would be a bit of work to back-port the older versions into this framework, but I suppose one could create branches on the spec repo named after the older versions and then configure readthedocs to build the older versions from these branches, specifically