Transferred from internal JIRA issue originally created 2022-04-04.

It's difficult for authors to maintain intro/notes pages since they require a special name based on the resource id – and the pages have to be renamed if the id is changed. It would be better to have the intro/notes associated to FSH definitions by name/id in text (rather than special file naming conventions). This allows for easier refactoring – and SUSHI can warn when it detects pages that refer to a non-existent definition.