Make most functions internal

[pbchase]

• Make most functions internal. See https://forum.posit.co/t/how-to-include-internal-functions/38654/1. I elected to replace @export with @keywords Internal. The function is not exported, the man/*.rd files are created, but the man pages for the internal functions are not indexed.
• Make function documentation more consistent. While I was looking at the function docs I saw a lot of self-consistency errors. I don't think I fixed all of them, but it's better.
• Make minor updates to getting_started.Rmd. There were typos in this doc.