Add links to all specs available on collector page

While trying to review collector#40016 (closed) I was hunting down references for all the specs involved. It turns out they are all over the place and it's hard to keep track of them. On collector.html we link to some of those specs, which is good. However, we should collect the links to all the missing specs and add them there, too, so we have at least one canonical place where all specs are just one click away.

Oh, and while we are at it we should replace those old gitweb links.