Document new "Automatic" configuration option.
This commit updates the help page 'email-notification.md' to reflect
the change in zulip#24075 from a checkbox to a dropdown for the
'Include organisation name in subject line' setting.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
David Rosa
Prakhar Pratyush
Lauryn Menard