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ARROW-15328: [C++][Docs] Streaming CSV reader missing from documentation (#14452)

Updates the C++ CSV reader docs to include documenting and an example of the streaming CSV reader (StreamingReader), as per the suggestion in [ARROW-15328](https://issues.apache.org/jira/browse/ARROW-15328). 

@ westonpace could you look at this and let me know if this is what you were thinking?

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>