[R] Add map_batches examples from vignette

apache/arrow#11894 introduces a section of datasets vignette showing how to randomly sample a dataset and compute aggregate statistics without loading the entire dataset into memory. These might be good examples for the cookbook as well.

Start by reading apache/arrow#11894 and the datasets vignette section covering random dataset sampling and aggregate statistics. Find the cookbook's R examples entry point, add corresponding map_batches examples, and confirm they demonstrate the workflow without loading the entire dataset into memory.

Written by the indexing model from the issue text.