docs: added pyiceberg to pyarrow conversion documentation

[Rashampreet4114]

Rationale for this change

This PR adds documentation for PyIceberg ⇄ PyArrow datatype mapping /conversion table
The Iceberg spec currently defines type mappings only for Avro, Parquet, and ORC.
This change fills that gap by providing a clear reference for Python developers working with PyIceberg and PyArrow.

The documentation:

• Lists all supported PyIceberg → PyArrow conversions, derived from the _ConvertToArrowSchema visitor class.
• Provides the natural PyArrow → PyIceberg reverse mapping for round-tripping.
• Highlights important details such as handling of field IDs, documentation metadata, decimal precision, large string/binary handling, and timestamp precision/timezones.

Are these changes tested?

No

Are there any user-facing changes?

Yes.

• New Markdown documentation: pyiceberg_pyarrow_mapping.md (with aligned tables and Python type classes).
• No changes to runtime behavior—purely documentation.

Closes #2226