Improve simulation templates

[jonathanolson]

@veillette had some excellent suggestions for improving the simulation templates in the discussion at https://github.com/orgs/scenerystack/discussions/143:

When selecting the 'application' option, users are greeted with a nice animation that displays "Your content goes here." This serves two valuable purposes - it confirms that the installation works properly, and provides an easy search term to locate the correct file to begin editing. However, when selecting 'simulation,' users see only a white page initially, with navigation elements loading after a delay. It's not immediately obvious where to add code (simScreen? simModel?). Consider adding a similar starter animation for simulations that could guide first-time developers to the right files. While removing this animation might become repetitive for developers that use this repeatedly, simulation development typically takes significant time, so I think this convenience outweighs any minor inconvenience.
You could also some information at the end of the script with a URL, pointing them to a webpage where they can find more about how to proceed.

Non-sim templates have a bit of that (but I wouldn't call it a "nice animation").

[jonathanolson]

In the above commits, the minimal content placeholder from the scenery templates was added to the sim templates (since it was easy, and is an improvement).

@brettfiedler do you have ideas on what a better placeholder content would be? Potentially something like the accessible-interaction demo biker?