Introduce new change set type: documentation

[release-notes]

Introduce new change set type: documentation

[alrik]

@derlarsen @baranga @IjzerenHein do you have any feedback on this?

[baranga]

@alrik Could you provide a example of usage?

[alrik]

I'd use it for statements like these:

title: Release Notes of an awesome project
releases:
- version: Unreleased
  added:
  - Add some fancy feature.
  docs:
  - Add an [example](examples/fancy-feature/) on how to use fancy feature.
  - Fixed api status code description of some legacy feature.
  - Setup code of conduct and introduce getting started tutorial.
  - Update license year in all code doc block annotations.