docs: note mxpython for Motrix visualization on macOS

[TATP-233]

Summary

• Document that MotrixSim native renderer commands on macOS / MacBook must use uv run mxpython instead of uv run python.
• Add the guidance to README.md and the zh_CN getting-started, simulation-backends, training, and G1 motion-tracking docs.
• Clarify that non-rendering Motrix training can still use uv run python ... training.no_play=true.

Linked Work

• Issue: N/A (direct docs update request)
• Milestone: N/A

Validation

• make check
• uv run pytest -m "not slow"
• Additional task-specific validation listed below

Commands actually run:

uv run pytest tests/scripts/test_check_docs.py -q
make check
make test

make test ran locally on macOS and failed on an existing render backend expectation unrelated to this docs-only change:

tests/utils/test_render_many.py::test_resolve_gl_backend_uses_egl_when_probe_succeeds
AssertionError: assert 'glfw' == 'egl'

Impact

• Backend impact: motrix
• Platform impact: macOS
• Training effect expected: no

Artifacts

• W&B: N/A
• benchmark result: N/A
• video / screenshot: N/A
• ONNX / checkpoint: N/A

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly