Add workflow to verify release notes are formatted correctly when added in PRs

[silvanocerza]

The release process can fail if the release notes yaml are not correctly formatted.
An example failure: https://github.com/deepset-ai/haystack/actions/runs/10669412195/job/29571286619#step:6:111

This is annoying as we usually discover this issue when drafting a new release, that slows down the whole process.

We should add a workflow that checks the release notes files added in a PR are formatted correctly to prevent this kind of issues to block the release process.