Add PowerShell to CI variable docs

What does this MR do?

Add documentation for working with windows batch and PowerShell based runners

Why was this MR needed?

Documentation was unclear when setting up a c# .net application with a runner on a windows machine.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1309