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ARROW-15091: [C++][Doc] Document nodes in C++ streaming execution engine

This PR's objective is to create a detailed documentation on using ExecPlan and available options. The task is divided into two sub-components. The first component is to work on a set of examples showing how to use these options with sample data.

## Examples

- [x] Source-Sink
- [x] Scan-Sink
- [x] Scan-Filter-Sink
- [x] Scan-Project-Sink
- [x] Source-Aggregate-Sink
- [x] Scan-ConsumingSink
- [x] Scan-OrderBySink
- [x] Scan-HashJoin
- [x] Scan-SelectSink
- [x] Scan-Filter-Write
- [x] Scan-Union-Sink

## Documentation

- [x] General Description
- [x] Option Description with code snippets

## Notes

Some of the util functions were referred from the `test_utils`.
There is a potential duplication of code due to that. Need
feedback on best way to handle this.

Closes #12033 from vibhatha/arrow-15091

Lead-authored-by: Vibhatha Abeykoon <vibhatha@gmail.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>