`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 https://github.com/Yu-Group/simChef/issues/177 as well.