(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.
fyi - there are a couple of other proxy-related http settings documented on https://docs.puppetlabs.com/references/latest/configuration.html - `http_proxy_host` and `http_proxy_password` - that Puppet Server does not support in the context of `Puppet::Network::HttpPool` connection requests.  It did not appear to me, though, that these two settings are honored by the Ruby master either when making outgoing requests to an HTTP report processor.  Since this is intended to be a differences doc, I didn't bother documenting those here.