Add .github/workflows/product-map.yml

[koppor]

This adds generation of maps for https://product-map.ai/

It tries to use https://github.com/product-map/product-map-action.

This is #research. Lets see, how everything goes.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

The build for this PR is no longer available. Please visit https://builds.jabref.org/main/ for the latest build.

[koppor]

Let's try how this works -- I think, if contributors submit something, the action either fails or silently does not update README.d