Prerequisite: Start with what we have

Curently we document the setup of disconnected environments and image mirrors in the following places:

• Install MCE in disconnected environments

• Custom clusterimagesets in disconnected environments

• AgentServiceConfig in disconnected envirionments

Describe the changes in the doc and link to your dev story

The current documentation doesn't describe how to mirror also image signatures, and setting up the mirrors without signatures generates problems during installation that are currently solved disabling signature checking (see MGMT-13934). That results in a potentially insecure setup. It would be much better to update the documentation to explain how to mirror also the signatures.

Note that `Documentation` may not be the right component for this, as the process to mirror images may need large changes and different tools.