CHANGELOG: add release notes for upcoming version

Reference issue

Resolves #2907 (closed), #2792

What does this implement/fix?

* Adds a CHANGELOG.md file, customised from Keep a Changelog

• Updates the CHANGELOG file with notes for the upcoming release

Additional information

Some changes are missing from the changelog, due to the time between when the notes were assembled and when this MR was opened. It can be edited in the future.

If I have time to do that, I'll add a script for updating the changelog automatically (it would parse the last heading in the file to get the previous version, get the new version, and update the [Unreleased] heading). Perhaps we should add a CI step where we check if someone has added their change to the changelog with their MRs to help collect release notes in the future @cantonios?