arrow

ARROW-15814: [R][DOCS] Improve documentation for cast()

The existence of `cast()` is mentioned in the data type documentation, but since it is not a stand-alone function, there is no `cast()` reference page, and users have had no way of knowing how to use it.
It was especially difficult to know that it could be used in a dplyr query.

Include usage examples of `cast()` during the data type help pages to give users hints on how to change the type of data contained in the arrow object.

Closes #12546 from eitsupi/r-docs-cast

Authored-by: SHIMA Tatsuya <ts1s1andn@gmail.com>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>