Improve documentation for multi-version so non-CI can fix if needed

[bms63]

I think the important point is that everything is well documented. For example, regarding documentation workflow we have It is not clear for which versions the workflow creates the documentation and how to add versions which are not created by the workflow.

@bundfussr : you can find a detailed documentation here : https://github.com/insightsengineering/r-pkgdown-multiversion - you can see that the argument branches-or-tags-to-list refers to the versions dropdown - Admiralci is mostly using some more generic actions (that can be used for any other R package) that's why not every documentation can be find directly inside admiralci repo (What we can do is quickly add a link to the external github actions documenation)

Yes, if an external action is used, a link to its documentation should be provided. What still is not clear to me if the documentation action just updates the "Versions" menu or if it also updates the folders in gh-pages.

Originally posted by @bundfussr in pharmaverse/admiral#2103 (comment)