Explicit documentation on testing/upgrade path for Layer0 instances

[diemonster]

We currently have good developer docs that outline how to update the API and runners for a given deployed Layer0 instance, but we should expand this section to provide more details such as:

• Scenarios (i.e. using the same layer0 instance for testing vs making multiple new layer0s, speed, etc)
• Requirements (i.e. docker)
• Examples (flow.sh or using l0-setup upgrade manually)
• Troubleshooting (i.e. docker push failure)

Overall, updating is a really critical part of how to dev with Layer0, but it's a sub-section in the docs when it should be a main section. We should also reference this doc in https://github.com/quintilesims/layer0/blob/develop/.github/CONTRIBUTING.md .