Update more_features_borrowing.qmd

[kweav]

1. Long term, we'll want to point to ottrpal::borrow_chapter, but it's not ready for prime time yet
2. So, right now, we still want to say cow::borrow_chapter, but be ready for cow to be deprecated (with warning)
3. Want the default recommendation to be borrowing Rmd or qmd files, not the rendered docs/md files.
4. Borrowing from a private repository is not supported nor will it be supported, so need to clarify that.