Add example httpd vhost config file

httpd.conf is only a partial configuration designed to be included in to
an existing vhost directive. This provides a (nearly) drop-in vhost
configuration for httpd.

I've included all directives here, but we may want to include the existing
`httpd.conf` in to this file to keep current convention.

I haven't tested this yet, but it would be good to get a quick first glance
idea of whether including this could be useful.
Are reusers likely to use this?