Add documentation for air-gapped / fully offline FlowFuse deployments

### Description

**Summary**

We currently do not have documentation covering how to deploy FlowFuse in a fully air-gapped (offline) environment and what requirements, dependencies, and supported patterns so we can properly support this deployment model.

This is becoming a blocker for enterprise deals process.

**What we need from engineering**

Please define the minimum viable requirements to run FlowFuse in an air-gapped environment:
• Private container registry requirements (how images are delivered/updated)
• Node-RED / npm dependency strategy (offline node installs)
• DNS requirements (internal resolution expectations)
• TLS / certificate authority requirements
• Authentication options (SSO vs local, offline constraints)
• Networking assumptions (how components communicate internally)

**Goal**

Produce a clear requirements sheet that can be shared directly with a customer’s IT team, outlining:
• All required infrastructure
• All supporting systems/services they must provide
• What FlowFuse provides vs what the customer must own

Link to example: https://fathom.video/share/tR9McHcTBG9_YETCAuQVU8ks9QmkQrYf