Refresh devops-guide docs

[PierreBesson]

This documentation should be refreshed and linked from this repo : https://paritytech.github.io/devops-guide/kubernetes/testnetmanager.html

[PierreBesson]

@aardbol do you think you could pick this task ?

[aardbol]

Sure, I can pick it up later.

[PierreBesson]

The DevOps guide (https://github.com/paritytech/devops-guide) has to be refreshed and updated with proper examples and explanations for:

• Ansible role
• Node Helm-chart
• Testnet-manager