Improve Device Agent install instructions regarding /opt permissions

> Another pain point we discussed briefly on our Friday meeting was there is no mention of `/opt` being a restricted directory - where the OTC command fails due to permissions.

> We (me and ZJ) seen a user tripped up first hand with a MAC user at the ProveIt conf.
> We seen it with @ppawlowski on a Linux box in our Friday session

> should we call that out? Or is it for a later round of iteration (i.e. move the default installation dir to somewhere not affected by this (for example) `~/.flowfuse-device`)

_Originally posted by @Steve-Mcl in https://github.com/FlowFuse/flowfuse/issues/5377#issuecomment-2773214915_