Docs: FlowFuse Expert - document open-by-default and close-persistence behaviour

[allthedoll]

Description

Context

As of v2.29, FlowFuse Expert opens automatically when a user enters the immersive
editor. If the user closes it, that preference is persisted — Expert won't reopen
on their next visit.

This is a user-visible behaviour change that should be reflected in the Expert
and/or immersive editor documentation.

What to document

• Expert is now open by default in the immersive editor
• Closing it persists the closed state across sessions
• Users can reopen it manually at any time

Screenshot needed

Screenshot showing Expert open by default in the immersive editor.

Reference

• FF Expert: Open by default with close persistence flowfuse#6818

Epic/Story

FlowFuse/flowfuse#6818

Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

[hardillb]

closed via FlowFuse/flowfuse#7060