arrow

GH-40575: [Docs][Python] Added JsonFileFormat to docs (#40585)

### Rationale for this change

Docs for JsonFileFormat is missing in the documentation. In this PR, I am adding it to the documentation.

### Are these changes tested?

Tested minimally by building in local machine (installed pyarrow by `pip install pyarrow`, built the docs, and observed `pyarrow.dataset.JsonFileFormat.md` being generated.)

### Are there any user-facing changes?

No.

* GitHub Issue: #40575

Authored-by: arunppsg <arunppsg@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>