Document using NPM registry for FlowFuse Device Agent (air-gapped environments)

### Description

https://app-eu1.hubspot.com/contacts/26586079/record/0-3/252826054853

We are working with a customer running FlowFuse in an air-gapped environment who would prefer to use Amazon CodeArtifact as their npm-compatible registry rather than the built-in flowfuse/npm-registry.

Their main reason is operational: using the built-in registry would require them to modify every container image to include a filesystem-watching process to sync modules, which they would like to avoid.

Instead, they need like all Node-RED runtime containers and palette dependencies to be pulled from CodeArtifact, which is already approved and used internally.