Document the verified Windows PowerShell quickstart

[five0nit]

Problem

The README has a cross-platform development note, but first-time Windows contributors do not have one compact copy/paste example for running the installed CLI and the release validator in PowerShell.

Smallest useful outcome

Add a short PowerShell quickstart using the existing supported commands and path conventions.

Acceptance checks

• uses .venv\\Scripts\\python.exe and .venv\\Scripts\\brief2ship.exe correctly;
• includes install, doctor, --version, and scripts/validate-release.py;
• does not claim WSL, Bubblewrap candidate execution, or optional Trafilatura works unless tested;
• avoids administrator-only steps;
• documentation links and release validation pass.

Good first contribution

This should be documentation-only unless the verified commands expose an actual defect.