The embedded data source plugin allows you to enter source data directly into a migration configuration yaml file. And it works brilliantly for creating just a few content entities. (Or just one content entity, the default store, in this case.)
I played around with the Drupal "Book" core module for a while. But found it wasn't quite what I wanted. Here's a simple approach for building a section of pages with a table-of-contents that did work for me.