It turns out, the link to the quickstart still used the old 'master' branch. It was changed to '6.3.x-develop' a few months ago. The 'master' was removed and this link was never updated.

I backed out this change because, while it makes sense to use relative links across documents within a repository, I don't think it's a good idea to do so across different repositories, because it breaks forked and cloned versions of the repository. For example, I may have cloned one and not the other and the links won't work.

Anyway, I updated the link to use the correct branch to the numberguess quickstart with this commit: https://github.com/jboss-developer/jboss-developer-shared-resources/commit/8a75dc0de5e03096e7fcda13d8488ea2d00d99cb