cleanup windows install docs
What does this MR do?
- Add missing steps to Windows install details
- Use
gitlab-runner
rather thangitlab-ci-multi-runner.exe
in support of https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1195 - Document how to use with true service account (Built-in System Account)
- General cleanup
- Provide better link on how to run elevated command prompt