Make more CHANGELOG urls clickable (#984)

* Make more CHANGELOG  urls clickable

Extension of PR #977 to v0.4.0.rc1:

- Make callouts `loud`.
- Replace a warning paragraph by a callout.
- Use a reference link instead of an explicit url for @pmarsceill .
- Add link reference definitions for PRs and new contributors in v0.4.0.rc1.
- Replace inline urls by shortcut link references.

The changes to replace URLs by link references would be too tedious and error-prone to do completely manually for v0.4.0.rc1.
The following regexp replacements were applied to the sections of v0.4.0.rc1, using Atom:

- Copy a section of changes.
- In the copy, replace `.*(https://.*pull/)([0-9]+).*$` by `[#$2]: $1$2`.
- In the original, replace `(https://.*pull/)([0-9]+)` by `[#$2]`.

- Copy a section of new contributors.
- In the copy, replace `\* @([^ ]+) .*$` by `[@$1]: https://githhub.com/$1`.
- In the original, replace `@([a-zA-Z0-9]+)` by `[@$1]`.

Add (co)authors in [#578]:

* [@AdityaTiwari2102] made their first contribution in [#477]
* [@svrooij] made their first contribution in [#544]

[@AdityaTiwari2102]: https://githhub.com/AdityaTiwari2102
[@svrooij]: https://githhub.com/svrooij

To test:

1. Check that the CHANGELOG contents looks the same in the docs and in the repo (apart from the callouts) with clickable links to all PRs and contributors for v0.4.0.rc1.

2. Check that none of the lines with changes or new contributors have been removed.

* Revert to quiet callouts