Link to actual stabilization reports instead of just PRs that contain those reports

The section on [stabilization reports](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#write-a-stabilization-report), links to two PRs that supposedly contain those reports. The second of these PRs contains hundreds of comments and it takes quite a while to expand those comments and find the actual report. We should link directly to them.