Docs: Update docs to support private container registries

MR: Update docs to reflect workspaces private conta... (!172538 - merged)

Description

As per the plan in Provide support to use images from private cont... (&14664 - closed) , update the docs to support private container registries.

When this issue is closed, close out Remote Development Workspaces - Documentation s... (#441226 - closed) as well.

Acceptance Criteria

• Docs are added for the new agent configuration image_pull_secrets.
• Call out that updating this field will only affect new workspaces.
• Call out that the secrets need to be created in the Kubernetes by the user.
• Call out that the name in image_pull_secrets has to be unique even if they are in different namespaces - because they will be copied in the namespace of the workspace.
• Call out that if the secret is not found, it will not be copied.
• Call out that any updates to the underlying value of the references secret would be reflected eventually, not immediately (during next full reconciliation between agent and rails).