GH-46853: [Python][Docs] Document preserving leading zeros in CSV

[kalyanamdewri]

Summary:

• Document that quoted numeric CSV values are still subject to type inference.
• Add a doctest-backed example showing how ConvertOptions.column_types preserves leading zeros.
• AI assistance was used for issue triage and drafting this documentation example.

Validation:

• /tmp/arrow-doccheck-46853/bin/python -m pytest --doctest-glob="*.rst" docs/source/python/csv.rst -q — passed, 1 passed
• git diff --check HEAD~1 HEAD — passed

• GitHub Issue: [Python] read_csv converts strings with leading zeros to integers #46853

[github-actions]

⚠️ GitHub issue #46853 has been automatically assigned in GitHub to PR creator.