Docs: Add use case to docs for system_id and reusing configurations
Problem to solve
The documentation about system_id explains how the id is generated and how GitLab Runner uses it, but a real use case would give users more context about how they can use system_id works in the new runner registration workflow, especially for configuration reuse.
Further details
Proposal
In the https://docs.gitlab.com/runner/fleet_scaling/#reusing-a-runner-configuration, add a real world use case of the system_id and how it's used and why.
Who can address the issue
Other links/references
Edited by Fiona Neill