Add preview of rendered report on documentation website

[Bisaloo]

sivirep currently provides a very nice looking report and it would be good to already display it to potential users when they discover the package.

A good way to do this could be to add a copy of the pdf on the pkgdown website or/and a screenshot in the README.

I can try to submit a pull request if you agree with this idea but you're not sure how to implement it.