Update Project Page to clarify the differences between contrib workflow and core

With the new "workflows" and "content moderation" modules in core, people will be searching for workflow notifications, and may land here before realizing that this is for the contrib Workflow module, and that this module is incompatible with the new core modules.

It would be greate to add a big notice on the project page that clarifies this and points people to Content Moderation Notifications → module, which works with core.

Thanks!