GH-45644: [Doc][Python] Document timezone loss when converting timestamp arrays to NumPy

[alex-anast]

Rationale for this change

NumPy's datetime64 type does not support timezones. When converting a timezone-aware Arrow timestamp array to NumPy via to_numpy(), the timezone information is silently dropped. This behaviour is expected but undocumented, which can surprise users (see #45644).

What changes are included in this PR?

Adds a "Timezone-aware Timestamps" subsection to docs/source/python/numpy.rst that:

• Explains the timezone loss when calling to_numpy() on tz-aware timestamp arrays
• Shows a code example demonstrating the behavior
• Documents two alternatives: to_pandas() for tz-aware Series, and to_pylist() for Python datetime objects with tzinfo

Are these changes tested?

Documentation-only change. All code examples were verified against pyarrow 24.0.0 and sphinx-lint passes clean.

Are there any user-facing changes?

No behaviour changes. This adds documentation for existing behaviour.

• GitHub Issue: [Doc][Python] Timestamp with tz loses its time zone after to_numpy #45644
  Closes [Doc][Python] Timestamp with tz loses its time zone after to_numpy #45644

[github-actions]

⚠️ GitHub issue #45644 has been automatically assigned in GitHub to PR creator.

[github-actions]

⚠️ GitHub issue #45644 has been automatically assigned in GitHub to PR creator.