[R] Add map_batches example from vignette to Cookbook

In [ARROW-14029](https://issues.apache.org/jira/browse/ARROW-14029) we are adding an example of using `map_batches()` to sample data and compute aggregate statistics without having the load the whole dataset into memory. We should add these to the cookbook as well.

Start by reviewing ARROW-14029 and the map_batches() example from the referenced vignette. Add the sampling and aggregate-statistics example to the R Cookbook, then verify that the example is runnable and explains processing without loading the whole dataset into memory.

Written by the indexing model from the issue text.