Improve documentation and explain how to use multiple runners

[andreyolv]

Improve documentation and explain how to use multiple runners (helm chart gha-runner-scale-set). Clarify the following points:

• What are the best practices for deploying multiple runners?
• Does each runner need to be in different namespaces or not?
• If you need different namespaces, is it necessary to install the gha-runner-scale-set-controller chart in each namespace?
• Examples of how to divide runners,
  • by different operating system versions: ubuntu 18, 20, 22
  • by different resource sizes: small, medium, large
  • other examples of good practices and common uses.
• What is the relationship between labels and multiple runners. And if applicable, how to use them to better organize multiple runners?
• What is the relationship between runnergroups and multiple runners. And if applicable, how to use them to better organize multiple runners?