DEV: Ensure the REL commit message is consistently created

[MartinThoma]

Any commit message starting with REL: should have the structure:

REL: {{ semantic_version }}

## What's new

{{ CHANGELOG_ENTRY of that version}}

Desired Workflow

In order to create the release (REL) commit, I run:

1. Run python make_release.py (currently: make_changelog.py). This (1) creates the changelog in an interactive way (2) bumps the version and (3) creates a (.gitignore-d) file RELEASE_COMMIT_MSG.md.
2. Run git commit -eF RELEASE_COMMIT_MSG.md && git push
3. The git tag is created by CI.

Acceptance Criteria

• make_changelog.py is adjusted so that it creates a RELEASE_COMMIT_MSG.md of the described format
• Add RELEASE_COMMIT_MSG.md to .gitignore
• Adjust the release documentation: https://pypdf.readthedocs.io/en/latest/dev/releasing.html

Alternatives

I thought about using a prepare-commit-msg git hook (docs), but that seems more complicated.