cd: auto-generate release notes for GitHub release page

[lukpueh]

Description of issue or feature request:
#1946 adds a CD workflow to release build artifacts on PyPI and GH upon successful completion of the CI workflow for a pushed release tag.

The PR also replaces the manual process of adding release notes to the GH release page with an automatically added link:

python-tuf/.github/workflows/cd.yml

Lines 83 to 86 in b99d043

	body: 'See [CHANGELOG.md](https://github.com/' +
	context.repo.owner + '/' + context.repo.repo + '/blob/' +
	'${{ github.event.workflow_run.head_branch }}'+
	'/docs/CHANGELOG.md) for details.'

Current behavior:
Auto-generated GH release pages only link to CHANGELOG.md.

Expected behavior:

Consider auto-generating more meaningful release notes (see GH docs)

[Nageshbansal]

hey @lukpueh , Is this issue resolved? if it isn't I would like to work on it.

[lukpueh]

Thanks for volunteering, @Nageshbansal! I'm definitely curious to see what auto-generated release notes would look like for this project.

[jku]

I think we're doing OK now: there is a script that gets the correct snippet from the Changelog file and puts it in the GH release