allow manual deploy of pkgdown website

[cderv]

@rich-iannone This will help redeploying without commiting to main

Thanks

[cderv]

I took the opportunity to update the workflows also to latest version of r-lib/actions following what I usually use.
No big deal, I was just going through some update on our workflow and applied the same here, as I saw Ubuntu 16-04 was still used but it is unavailable on github action now I believe.