Update Config chapter for recent Nushell changes (e.g., user autoloads)

[NotTheDr01ds]

Updates the Configuration Chapter of the Book for recent updates, mainly user autoload directories.

[blindFS]

I have a question about this feature: should those files in autoload dirs loaded with nu --config minimal_config.nu? Or is there any flag to disable them?

Actually I want to keep a minimal config file for the lsp server, that's the only reason keeps me from switching to this fancy setup.

[NotTheDr01ds]

There's no flag today, but I do something similar with:

let temp_home = (mktemp -d)
$env.XDG_CONFIG_HOME = $temp_home
$env.XDG_DATA_HOME = $temp_home
nu --config minimal_config.nu
# Optionally
#rm -r $temp_home

There's a somewhat related Discord drawing-board where I discuss flags and autoloads, but more so forcing them to load when they wouldn't otherwise be.

If we wanted to disable them, I'd probably try to rewrite the --no-config (-n) option to allow --config-file and --env-file to override it. Today it's the opposite.

But I still think the entire startup needs some cleanup. That's mainly why I haven't gotten around to it yet.