(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.
(SERVER-103) Added blank lines between setting names and fixed link

This commit adds some blank lines between settings in the
`webserver.conf` example and fixed the link for the `X-Client` headers
to point to the new doc that SERVER-104's PR will introduce.
Merge pull request #241 from camlow325/feature/stable/SERVER-103-external-ca-docs

(SERVER-103) External CA documentation