Document how to enable FlowFuse Expert & Assistant for self-hosted customers

[dimitrieh]

Summary

Add a handbook page that walks through the process of enabling FlowFuse Expert and Assistant on self-hosted enterprise instances. This should serve as a runbook that any engineer can follow to action an enablement request end-to-end.

What to cover

• Step-by-step instructions for enabling Expert on a self-hosted instance
• Step-by-step instructions for enabling Assistant on a self-hosted instance
• The flowfuse.yml configuration the customer needs to apply
• Network requirements (connectivity to FlowFuse API)
• Token generation and provisioning steps
• How to verify enablement is working correctly

Why

This process currently relies on a small number of people who know the steps. Documenting it means any engineer can handle enablement requests without delay or context-switching from the people who set it up originally.

[knolleary]

I created a draft PR to update the existing documentation before this issue was raised. It is draft as it is pending the subtasks of https://github.com/FlowFuse/CloudProject/issues/1215 being completed as they modify bits of the process.

This issue has a broader scope, so need to review and write the delta.

[allthedoll]

Status: Blocked, awaiting work. Expected to be complete after Traefik ➡️ work by @Steve-Mcl @hardillb ➡️ this.