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ARROW-9914: [Rust][DataFusion] Document SQL Type --> Arrow type mapping

DataFusion uses SQL data types for its query language, but Arrow types for its plans and execution. We should document the mapping of SQL types to Arrow types.

I wanted to refer to this table while working on the Timestamp proposal, so I figured I would write it down :)

Closes #8111 from alamb/alamb/datatype-docs

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Paddy Horan <paddyhoran@hotmail.com>