docs: add docs on small file compaction with optimize #4603

Job	Run time
Python Benchmark	10m 7s
lint	4m 25s
Python Build (Python 3.10 PyArrow latest)	8m 0s
Python Build (Python 3.7 PyArrow 8.0.0)	2m 10s
Running with Python 3.7	7m 36s
Running with Python 3.8	7m 42s
PySpark Integration Tests	10m 24s
Running with Python 3.9	3m 59s
Running with Python 3.10	4m 1s
Running with Python 3.11	3m 51s
	1h 2m 15s

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