Fix timezone type annotations for now helpers

[kokokoXUY]

Fixes #783.

The top-level now() annotation excludes datetime.timezone and zoneinfo.ZoneInfo, even though DateTime.now() and the runtime already accept datetime.tzinfo. The same mismatch affects today(), tomorrow(), and yesterday().

This aligns the four public annotations with their runtime behavior, adds typed regression coverage for standard-library UTC and ZoneInfo("Europe/Paris"), and updates the README and instantiation guide. No runtime behavior changes.

Validation on Windows CPython 3.12.7:

• Pure-Python suite (PENDULUM_EXTENSIONS=0): 1847 passed, 5 skipped.
• Focused mypy on tests/test_main.py: passed. Before the change it reported four incompatible-argument errors for these helpers.
• Ruff 0.14.11 check and format check: passed.
• Full mypy: one pre-existing unused-ignore error in src/pendulum/tz/timezone.py:65; reproduced on unchanged master.
• The native Rust extension was not tested locally because this machine has no Rust toolchain; CI covers that matrix.

AI assistance: Codex helped investigate, implement, and test this change. I reviewed the resulting diff and test results.