GH-30800: [Python][Docs] Document partition fields with explicit dataset schemas

[kalyanamdewri]

Summary:

• Document that explicit dataset schemas for partitioned datasets must include partition fields used in filters or projections.
• Add a doctest-backed example showing a Hive partition field used with count_rows.

Validation:

• /tmp/arrow-doccheck-30800/bin/python -m pytest --doctest-glob="*.rst" docs/source/python/dataset.rst -q — passed, 1 passed
• git diff --check HEAD~1 HEAD — passed

• GitHub Issue: [Python][Docs] Opening a partitioned dataset with schema and filter #30800

[github-actions]

⚠️ GitHub issue #30800 has been automatically assigned in GitHub to PR creator.