other: Point to ChangeLog file for release notes
While at it, add 'details' type to urls, see https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/#release (but no idea, what's a detailed release note link)
We didn't manage to add the release notes to the previous location in time. That's why, let's point to ChangeLog, where everything is done automatically.