Add versioning

[matmaxgeds]

In order to add embedded codelists it would be useful/essential to have some form of versioning. Vuepress doesn't seem to have this as of Nov 2019 but there is an open issue on it with active discussion which links here and here as two options. I also wonder if we couldn't 'cheat' by using subfolders in /docs, and then using hierarchical/collapsible sub menus.....would mean much duplication, but only visible in the UI if you expanded multiple IATI version menus at the same time so probably acceptable?

Also, my instinct would be to version everything, even if we just give some of them the version 'current' for now, as I think it would help a lot of people if some of the currently non-versioned codelists were versioned by date e.g. you could pick a version of a non-embedded codelist that matched the published date of the IATI data you were using it with as I suspect this might increasingly be a problem with using older IATI data.

[matmaxgeds]

copying here from @markbrough "So the non-embedded codelists are versioned in the underlying Github repo, but hopefully this shouldn't be a problem in future anyway, as we are trying to maintain a principle of never deleting a code from a non-embedded codelist and just tracking them as withdrawn. So you should always be able to use the most up to date version of a non-embedded codelist (though there are probably some historical codes that are not currently included as I am not sure how far back tracking of the withdrawn codes goes). Agree it would be cool to include embedded codelists too -- I think for this we should probably switch away from vuepress to nuxtjs as it gives us quite a lot more flexibility