Add/fix windows install instructions (#6579)

## Why are these changes needed?

Install instructions for Windows are missing or incorrect

## Related issue number

closes https://github.com/microsoft/autogen/issues/6577

## Checks

- [ ] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
<https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md> to
build and test documentation locally.
- [ ] I've added tests (if relevant) corresponding to the changes
introduced in this PR.
- [ ] I've made sure all auto checks have passed.

Co-authored-by: Victor Dibia <victordibia@microsoft.com>