Register MAgent2 environments with PettingZoo

[entropy-z0]

Description

Register all six MAgent2 environments with PettingZoo's environment registry
for both the AEC and parallel APIs.

After importing magent2, users can create environments through the new
Gymnasium-style API:

from pettingzoo import make
import magent2

env = make("aec", "magent2/battle-v4")

This also:

• updates the minimum PettingZoo version to 1.27.0;
• preserves the existing module-based constructors for compatibility;
• documents the new creation API;
• adds coverage for all registry entries and both environment interfaces.

Fixes #101

Validation

• pre-commit run --files magent2/__init__.py pyproject.toml tests/test_magent2.py docs/introduction/basic_usage.md
• resolved all 12 registered entry points
• git diff --check