Aim extra config reference link at 3.5.latest
Remove stub "dirs_environments" page

This turned out to not be necessary, and might have been confusing if I had
completed it.
Set canonical URL of /guides/configuring.html to /puppet/latest/reference/config_about_settings.html

This page is now basically obsolete, but since its replacement applies strictly
to Puppet 3.5, I'm not ready to delete it yet. Setting the canonical link will
serve as a soft-redirect for the time being, and will start draining its
google-juice toward this group of better pages.