[CHORE]: Add CI workflow to make sure changelog gets updated with every PR

[emilykl]

Link to issue

Closes #5723

Description of change

Add a CI job to check that CHANGELOG.md is updated for every PR, unless the no-changelog label is added to the PR.

This implementation is pretty naïve; it only checks that CHANGELOG.md has been modified, not whether the new entry follows the correct format. Feel free to complexify in the future.

Demo

❌ Run failed (CHANGELOG.md not updated):
https://github.com/plotly/plotly.py/actions/runs/34384286059/job/102576548616

⚪ Run skipped (no-changelog label added):
https://github.com/plotly/plotly.py/actions/runs/34384435370/job/102577056340?pr=5724

✅ Run passed (CHANGELOG.md updated with dummy entry for demo purposes):
https://github.com/plotly/plotly.py/actions/runs/34385144520/job/102579417311?pr=5724

Guidelines

• I have reviewed the pull request guidelines and the Code of Conduct and confirm that this PR follows them.
• I have added an entry to the changelog if needed (not required for documentation PRs).