[R] Add recipes on how to use s3/gs_bucket

The documentation for these is not great as `?s3_bucket` doesn't really explain anything as the actual docs are in `?S3Filesystem` which is also missing usage examples.

My use case: I wanted to read a bunch of csv that where in a bucket, I did not find an easy way to just get the list of files in a "dir" as full uris I could then use with `read_csv_arrow` (which I would have expected to work with the bucket object directly tbh; something like `read_csv_arrow(bucket$cd("csv_dir"))`) So I had to paste the file list onto the s3 uri to be able to load them with `read_arrow_csv`.