[R] Add content on Tables vs. Datasets

There are fundamental differences between working with Tables/InMemoryDatasets and file-based datasets. There should be content about working with datasets which has an intro covering those differences, and recipes for working with them.

Possible topics:
* Adding new data to a dataset
* Link to reading/writing sections
* Mention of reading only reading in relevant data

This content potentially could be part of other chapters, but is definitely needed

No file, test, or entry point is named. Start by locating the reading/writing sections referenced in the issue and review nearby dataset content to choose where the new material belongs; done means the cookbook explains Tables/InMemoryDatasets versus file-based datasets and includes the requested recipes and links.

Written by the indexing model from the issue text.