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ARROW-16006: [C++][Docs] Provide row conversion example for dynamic schemas (#12775)

This PR is meant to improve the ease of converting between Arrow tabular structure and row-based data. It adds an example of implementing and using the row converters to convert between vectors of `rapidjson::Document` and Arrow tables, highlighting some common patterns and important utilities provided by Arrow C++.

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>