create_sim: improvements to docs and vignettes

[jpdunc23]

Motivated by #175, the documentation and vignette for create_sim could be improved. Here are some specific items:

• Change "character" to "string" in the description of the path arg.
• Change "high-power computing" to "high-performance computing" throughout.
• Add examples with file trees for various combinations of the args. Could some or all of the vignette be incorporated into the docs?
• Incorporate create_sim() in the Getting started with simChef vignette.

More generally, we should emphasize create_sim() as an optional first step to setting up a new simulation project and clarify our basic integration with renv and git.

See #177 as well.