Improve release workflow

Research how other organisations manage their release workflows - in particular how different branches are used (development, release candidate, release, fix, etc.) - and come up with improvements to our own process.

Some other related tasks:

• Improve release notes instructions. Mention Milestones and how they should be used.
• Draw a flowchart with all the steps that need to be followed during a release.
• Consider having a "release candidate TAG" (without triggering the automatic release workflow).