Minor: Split physical_plan/parquet/mod.rs into smaller modules (apache#10727)

* Minor: Split physical_plan/parquet/mod.rs into smaller modules

* doc tweaks

* Add object store docs

* Apply suggestions from code review

Co-authored-by: Ruihang Xia <[email protected]>

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Co-authored-by: Ruihang Xia <[email protected]>

Author: 2 people