readme: update Python manual install instructions for Windows

[MikeMcC399]

• closes Outdated Python 3.14 manual install instructions on Windows #3264

Checklist

• npm install && npm run lint && npm test passes
• documentation is changed or added
• commit message follows commit guidelines

Situation

Python instructions to install on Windows changed with the release of Python 3.14 in October 2025.

Python 3.14 documentation says:

To obtain Python from the CPython team, use the Python Install Manager. This is a standalone tool that makes Python available as global commands on your Windows machine, integrates with the system, and supports updates over time. You can download the Python Install Manager from python.org/downloads or through the Microsoft Store app.

Python 3.14 is not available as a separate package in the Microsoft Store, unlike Python 3.12 and 3.13.

The Python Install Manager can be used to install 3.14 and other versions.

Description of change

In the README > On Windows section change the manual install instructions to:

Follow the instructions in Using Python on Windows to install the current version of Python.

[MikeMcC399]

Preview

https://github.com/MikeMcC399/node-gyp/blob/python-install-windows-doc/README.md#on-windows