Add optional presets to demo, like OpenAI

[ccmaymay]

Extending #56: add a few configuration presets to demo, as in the OpenAI playground. Use a simple default that's a good starting point for several tasks (don't surprise the user). This would be a good time to switch to our own handwritten examples for copyright/licensing purposes.

This feature might also lighten the load of #45: We can perhaps avoid some abstract explanation by pointing to concrete examples presented in the main interface.