Update the vignette with better demo code

[pbchase]

Update the Getting Started vignette based on the content created for https://github.com/ctsit/participantFlowDiagram_presentation. Consider creating a second vignette that discusses how to make the participant_level_progress object. Consider providing working example code.

• Copy demo.R to this repo to provide a more complete example of how to use the code.
• Copy any portions of participantFlowDiagram-presentation.qmd that are not addressed in this vignette.