feat: show chnagelog in in-app update

[ShlomoCode]

At the moment the user only knows that there is an update, but he does not know what is new/fixed. This PR displays an automatically updating changelog within Sparkle's update window.

1. I created CHANGELOG.md using https://github.com/rhysd/changelog-from-release CLI
2. I set up their workflow, to automatically update the file on each new release
3. I set sparkle:releaseNotesLink to the address of the changelog file on DockDoor's github pages

expected result:

[ejbills]

Hey, this is awesome, thanks!!

[illavoluntas]

Could the changelog be dark to have continuity (in the case of the screenshot) ?