With https://bugs.kde.org/show_bug.cgi?id=388293 it has been reported that the mentioned links are gone. I fixed that temporary with https://cgit.kde.org/websites/umbrello-kde-org.git/commit/?id=b6a94baa784c2433bac1f93bdea8249a783e0664, but the question is how to proceed in the future.
Some background: I learned that for KF5 based packages 'stable5' or 'trunk5' would be a replacement, but the stable Windows releases are still bound to KDE4 (because there are several blocking KF5 issues (https://bugs.kde.org/showdependencytree.cgi?id=380136&hide_resolved=1). I'm doing my best to fix these issues in my free time, but the time I'm spending to fix issues in the basic frameworks is lost to work on the application and there are several bugs waiting to be fixed. Using a shared branch for KDE4 and KF5 is a result of this conflict, because merging a KDE4 based branch into a KF5 based is known to be potentially buggy and requires significant fixing efforts on every merge (Try to merge branch Applications/17.12 into frameworks branch from umbrello git repo). The kmymoney project for example canceled merging from the stable KDE4 branch to the KF5 based master because of those merge issues.
With that in mind there is branch Applications/17.12 a stable KF5 branch for non Windows platforms and a stable KDE4 branch for Windows until KF5 libraries are ready to be used for stable KF5 umbrello. Therefore there should be a doc branch 'stable4' for Windows and 'stable5' for non Windows and 'trunk5' for KF5 based build on Windows ?