Document Expert & Assistant enablement process in handbook

[dimitrieh]

Summary

Document the FlowFuse Expert and Assistant enablement process in the handbook so that all teams have a shared understanding of:

• What "Assistant" vs "Expert" means internally (two separate implementations, one external brand)
• How to check what's enabled for a given customer (cloud and self-hosted)
• The process to request enablement for a self-hosted customer
• Network requirements for self-hosted customers
• Who owns each step of the enablement process

Why

There is currently no documentation for this process. Questions about enablement status require consulting multiple people across engineering. This should be self-serve for CS and sales.

Acceptance Criteria

• Handbook page exists explaining Assistant vs Expert distinction (internal naming, external branding)
• Enablement request process is documented (who to contact, what's needed, expected timeline)
• Network requirements and limitations are documented
• Page is linked from relevant handbook sections

[dimitrieh]

Fixed with #4725