cleanup windows install docs

Closed username-removed-5332 requested to merge bbodenmiller/gitlab-ci-multi-runner:patch-2 into master

What does this MR do?

  • Add missing steps to Windows install details

  • Use gitlab-runner rather than gitlab-ci-multi-runner.exe in support of

  • Document how to use with true service account (Built-in System Account)

  • General cleanup

  • Provide better link on how to run elevated command prompt

Replaces &