Document workflow for infrastructure provisioning

[fgiloux]

@Roming22 this is a follow-up of your PR #86. I feel that

• gitops/README.md should be extended to provide instructions on
  • the structure of our repo with the availability of kustomize base directories and their location
  • the standard approach for consuming it by either forking (especially in disconnected environment) or copying the sre directory
  • the way to create overlays using local base (fork) or remote base (copy)
  • the presence of examples under /doc/sre/examples
• . /README.md should introduce and link the gitops/README.md under "How do I start?"