Unified changelog format

[peel]

As of now we host several formats of the same change history to keep aligned:

• git log
• issues/milestone (source of truth for git log)
• CHANGELOG (a sorted git history with "closes" word removed)
• release notes page (similar to CHANGELOG but with headings and markdown format)

As well as:

• discourse post
• blog post

Who's the target audience?
How does that work with hosted docs?
How does that differ between libraries and end-user apps?
What can be generated?
What format shall we unify upon and how?

Adding not to forget about the question in the future.

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additional information:
https://github.com/release-drafter/release-drafter#readme