[Docs] Document the process of upgrading the Operator

Summary

We'll need documentation outlining the process to upgrade the GitLab Operator itself. This is different from updating the Chart version that the Operator deploys.

Items to cover:

• Operator upgrade process step-by-step
  • Via YAML manifest
  • Via OLM
• Considerations related to the chart version deployed when upgrading

Acceptance criteria

• Operator upgrade process is tested
• Operator upgrade process is documented