Make CHANGELOG urls clickable (#977)

* Make CHANGELOG urls clickable

- Add link reference definitions for PRs and new contributors in v0.4.0.rc2
- Replace inline urls by shortcut link references
- Add front matter so the page appears in the navigation
- Change the link to the CHANGELOG on the home page

The CHANGELOG contents should look the same in the docs and in the repo (apart from the callouts) with clickable links to PRs and some contributors.

The urls shown in the changes in v0.4.0.rc1 (and previous releases) haven't yet been made clickable – it would be too tedious and error-prone to do it manually.

* Make CHANGELOG urls clickable

- Add a callout