Add Windows as supported OS in install docs

What is the type of issue?

_No response_

What is the issue?

The page at https://github.com/openai/codex/blob/main/docs/install.md currently makes it sound like installing on Windows (without WSL) isn't an option, but the GitHub action does this and I found it just as easy to get started on Windows as Linux.

I think making it sound like Windows isn't supported might deter contributors from helping to fix Windows issues.

The only small tweak I needed to make to the codebase was in the justfile:

• remove set positional-arguments
• replace $@ with {{args}}
• add set windows-shell := ["powershell.exe", "-NoProfile", "-Command"] near the top

I'm happy to do a PR with both Windows install instructions and the justfile fix if desired.

Where did you find it?

_No response_