[Docs] Document Windows installation

[DecampsRenan]

Why

The new install method must be discoverable and trustworthy. README and the next release notes must show the PowerShell one-liner and any Windows-specific caveats so users land safely. Blocked by [Installer].

Topics

README

• "Windows (PowerShell)" section added under Getting Started with the one-liner
• Windows-specific behavior documented (e.g. PATH refresh after install)
• PowerShell version requirements stated

Caveats & troubleshooting

• Execution policy adjustment documented (Set-ExecutionPolicy) if needed
• SmartScreen warning documented with how to proceed
• Install location and uninstall steps documented