Fix broken links in Getting Started

[josephguillaume]

https://github.com/josephguillaume/hydromad/blob/HEAD/vignettes/hydromad.Rmd should not link to old pages on http://hydromad.catchment.org
http://www.nfrac.org/felix/papers/ is also broken.

The whole Getting Started page could also be revisited given the content in https://hydromad.github.io/index.html