Improve description of nightly build on download.slicer.org

[pieper (Steve Pieper (Isomics, Inc.))]

@lassoan

Should we add a note to the download page that nightly build is only recommended for developers until further notice?

@fedorov

Maybe we should add a link to the latest stable nightly?

I think these are both good ideas - I also wonder if we could add something like a “status” line to the download page that indicates what the overall developer feeling is about the nightlies. Right now the status would be “unstable because new features are being tested” but just before the release 4.10 it would be “pretty stable and contains many new features and fixes”. Right now users only get our opinions about the status if they ask or follow the forum closely.