investigate version drop-down for pkgdown website

[dragosmg]

similar to what {arrow} use to have or to what numpy has https://numpy.org/doc/stable/index.html

[dragosmg]

I like the numpy implementation. {arrow} used to have this, but then they switched to having a link to older pages in the footer:

The custom JS code used to power the dropdown selector interfered with the Bootstrap 5 {pkgdown} layout, resulting in the search functionality not working properly. Hence the decision to switch to the footer approach.

Upstreaming the feature to {pkgdown} was not successful (PR still open: r-lib/pkgdown#2072).

In conclusion, I'm not sure we will find a reliable way to do this before {pkgdown} is ready to handle multiple versions.

[dragosmg]

{admiral} have a Versions button with a dropdown menu, maybe that is more robust.
https://pharmaverse.github.io/admiral/cran-release/

[dragosmg]

https://github.com/insightsengineering/r-pkgdown-multiversion/tree/main