(SERVER-138) Created puppet.conf differences doc

This commit introduces a new document which contains some information
about the differences in supported settings from the `puppet.conf` file
for Puppet Server as opposed to the Ruby-based Puppet master.
Thanks, @ahpook.  Unless you think there's value in putting these few http settings into their own section, I might just fold them back into the previous list (in alphabetical order).  That's how the canonical puppet.conf settings page is laid out so might be good to just follow that pattern.