Support for navbar button tooltips in _pkgdown.yml

[manciniedoardo]

Disclaimer: I'm not sure if this is already possible, and if so please feel free to close the issue.

It would be great to add an option to easily add tooltips to navbar buttons. For instance something like this in _pkgdown.yml:

Resulting in:

[jayhesselberth]

This seems like a niche application but you can always add custom JS to a pkgdown site via pkgdown/extra.js.