Show old versions of documentation

[kamilzyla]

Goal

With #529 we now show the documentation for the latest release. It would be great if it was possible to access the documentation for older Rhino versions as well.

Approaches

It seems there is no common approach to this problem in the R community. Below is a list of resources which can help us to find a solution for our case:

1. Only keep old versions of function reference. See Shiny - Reference.
2. Keep old versions of the whole documentation site and replace the version badge with a dropdown selector. See Customising pkgdown with a version selector.
3. Use r-pkgdown-multiversion GitHub Action.
4. Use pkgdown Development mode.
5. Wait for more direct support from pkgdown (Consider readthedocs style handling of multiple versions r-lib/pkgdown#1373).