Reorganize use cases add new use cases

[JulieWinchester]

• Extends the number of use cases in the index to 9.
• Establishes simplest 3D use case as use case 6 (not 5 as before). Other more elaborate 3D examples are sub-headings from the simplest case since they mostly expand the astronaut scene.
• Adds use case 7 for two canvases nested in a parent canvas and adds a new manifest example demonstrating this. Other nesting examples don't be sub-headings, because they are very different from the use case example.
• Adds use case 8 for commenting annotations, and elevates the previous commenting on feature of painting to a full manifest as the use case example.
• Adds use case 9 for activating annotations, and elevates the light switch hidden/disabled example as the use case example. Also makes tab spacing in this example consistent with other examples in the index.