Add stable version example to docs

[sdruskat]

Related issues

Refs: #382

Describe the changes made in this pull request

• Add an example for a url identifier pointing to a stable release of the work

Review checklist

• [ ] Please check if the pull request is against the correct branch
  (format/schema/semantic documentation changes: develop; typos, meta files, etc.: main)
• [ ] Please check if all changes are recorded in CHANGELOG.md and adapt if necessary.
• [ ] Please run tests locally.

cd $(mktemp -d --tmpdir cff.XXXXXX)
git clone https://github.com/citation-file-format/citation-file-format .
git checkout <branch>
python3 -m venv env
source env/bin/activate
pip install --upgrade pip wheel setuptools
pip install -r requirements.txt
pytest
<do other things>

• [ ] Check place and content of documentation change