I am updating the documentation because now user will have to run the demo using a scene call "BootstrapDemo" as Active Scene, this new BoostrapDemo Scene is just a copy of a SceneTemplatedefined in the project, you can look the source template scene call "Bootstrap" and manage as you want to, so it's made to be reusable.
In this Bootstrap Scene you will be able to find the SceneLoader GameObject, which is a manager that load and unload Scenes from lists that you can manage, by default it will load the HDRP Template Demo scene.
If you want to use the scene manager for your own scenes, just drag and drop sceneAssets into a new created list, then use compnent's context menu then "populate SceneList Names", it will add the scenes to the build settings and make sure that the scene can be load at runtime as in Editor using stored names ref.