(SERVER-103) External CA documentation

This commit includes a new document which covers the use of an external
CA with Puppet Server.  This commit also adds a link to the new document
from a couple of existing documents.
You can have a file with `.key` as the file extension.  Our webserver logic doesn't care specifically what the extension is.  I went with `.key` in the example here because that's what the current "External CA" document for the Ruby master uses in the Apache VirtualHost configuration -- https://docs.puppetlabs.com/puppet/latest/reference/config_ssl_external_ca.html#puppet-master.