Refactor push rules docs

Achilleas Pipinellisrequested to merge
docs/push-rules into master

What does this MR do?

  • Add Overview and use cases
  • Remove image
  • Describe all available rules

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/2703

Edited by 🤖 GitLab Bot 🤖

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