Windows installation via pip/PyPI support in beta

Hi all!

I'm super excited to annonce that today's release of OpenSpiel 1.6.13 includes Windows support! 🎉

Thanks to the amazing contribution of @visheshrwl 👍 👍 👍 Windows wheels have now been built and uploaded to PyPI for Python versions 3.11, 3.12, 3.13, and 3.14. So, they should be available directly via `pip install open_spiel` on Windows. For more detail, please see the [Windows installation instructions](https://github.com/google-deepmind/open_spiel/blob/master/docs/windows.md).

Please be warned that is was lightly tested on Github Actions and is still in a beta phase. If you have a Windows machine, we would love some volunteer testers. All feedback is welcome. Please let's use this thread for any questions or issues that come up. Ideally, we will fix everything before the release of OpenSpiel 2.0. Thanks and enjoy!

Start with docs/windows.md and test `pip install open_spiel` on a Windows machine using Python 3.11, 3.12, 3.13, or 3.14. Record installation problems, questions, or other feedback in this issue; done means the beta installation path has been exercised and the results are documented.

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