This way it's easier to delete the entire .unreleased folder after the
changelog is generated, instead of enumerating every single changelog
file but skipping the templates.
This PR introduce the release notes header template using Jinja [1].
Also improved the script to merge changelogs to include the upcoming
.unreleased/RELEASE_NOTES_HEADER.md.j2 where we'll actually write the
release notes header for the next release.
The CHANGELOG.MD file contains the sections features, bugfixes, and
thanks. This patch adjusts the script merge_changelogs.sh to produce
the sections in the same order.
The changes in this commit
1. workflow action to check if the PR has its own changelog file in
".unreleased/" folder.
2. script to merge the individual changelog entries that can be copied
into the CHANGELOG.md file.
3. script to check the format of the lines in the change log file.
4. script to delete the individual changelogs, post release.
