Add a changelog

This MR adds a changelog.rst file in the root of repo and symlinks in the docs/source folder so it can be included in the documentation.

I'm hesitant about the format, but keeping it rst means we can more easily refer to other documentation page and include small code examples. Lots of other projects use a more simple markdown changelog

I added a checkbox for the merge request template, but we should all remain vigilant that we update this when creating a merge request. It is also important to replace the 'Next Release' placeholder with the actual version.