docs: Fast Refresh

[jaredcwhite]

I need to document the new Fast Refresh feature and explain some of the possible issues someone might run into that would require a stop/start. One could also just switch the feature off in the config (fast_refresh false), but that should be a last resort.

I'd considered making Fast Refresh an "experimental" feature that's opt-in for the initial v2.0 release, but I've already found it so-o-o useful on a number of my sites I've been testing with it, I think it'd be actually bad for it to be opt-in. I'd rather throw caution to the wind and collect bug reports right away and fix issues as quickly as possible, because Fast Refresh overall is so darn good. 🤩