Document runner managers and system_id
What does this MR do?
This MR updates the documentation in https://docs.gitlab.com/runner/fleet_scaling/ to cover runner managers.
Why was this MR needed?
Runner managers are not formally documented yet, so we don't have somewhere to point users to when we want to give them an overview of how to work with runner managers.
What's the best way to test this MR?
https://main-runner-4549.docs.gitlab-review.app/runner/fleet_scaling/#reusing-a-runner-configuration
What are the relevant issue numbers?
Edited by Pedro Pombeiro