composite repository points to released update site now on SourceForge
skip default deployment of Maven artifacts (we are not deploying to Maven repo)
upload Isabelle/Eclipse artifacts (p2 repo and standalone products) to SourceForge release section

Using `isabelleeclipse` project in SourceForge to host downloads since GitHub does not support it anymore;
Upload requires settings.xml with SourceForge authentication;
generate a p2 composite repository to point to the latest released `nightly` update site

The lightweight composite repository would be hosted on a web page pointing to update site in SourceForge file release system
info page for generated nightly composite repository (to be used with jekyll website)
updated to new download links
upload new nightly composite repository to GitHub pages
pointing to correct nightly build update sites
Merge branch 'sourceforge-releases'