Undefined variable $site_dir

[b-sharpe]

Describe the bug
The default includes settings file suggests using a "$site_dir" variable which does not exist, resulting in a undefined variable (and not included file). This is also referenced (but not used) in settings/config.settings.php so I'm assuming this is a forgotten relic.

To Reproduce
Install and add a settings file to the includes as suggested, using "$site_dir"