Use logo.svg (instead of logo.png) if present for package doc

[nr0cinu]

Hi,

right now, only logo.png is used for package documentation.

This could be amend, so also logo.svg will also be used. This would then bring it in line with https://pkgdown.r-lib.org/reference/build_favicons.html

I am happy to write a pull request for this too.

Best, Bela